intro-to-datetime.dd

Ddoc

$(D_S $(TITLE),

    $(P $(I by Jonathan M Davis))

    $(H2 Introduction)

    $(P In dmd 2.052, the module $(STD_DATETIME) was introduced. It will be
        replacing $(STD_DATE) entirely. As such, $(STD_DATE) is currently
        scheduled for deprecation. At a later date it will be deprecated
        (at which point, you'll have to compile with -d for it to work rather
        than simply having the compiler complain when you use it), and
        eventually it will be fully removed from Phobos. What this means is that
        all new code should be written to use $(STD_DATETIME) and that any code
        which currently uses $(STD_DATE) is going to need to be refactored to
        use $(STD_DATETIME) (unless you want to copy $(STD_DATE) to your own
        code and continue to use it as a non-Phobos module). This article
        attempts to familiarize you with $(STD_DATETIME) as well as give some
        advice on how to migrate code from $(STD_DATE) to $(STD_DATETIME) for
        those who have been using $(STD_DATE).)

    $(P $(STD_DATE) is essentially a C-based solution for dates and times. It
        uses $(D d_time) to hold time where $(D d_time) is a 64-bit integral
        value holding the number of milliseconds which have passed since
        midnight, January 1st, 1970 A.D. in UTC. C, on the other hand, uses
        $(D time_t) to hold time where $(D time_t) is an integral value holding
        the number of seconds which have passed since midnight, January 1st,
        1970 A.D. in UTC. Its size varies from architecture to architecture
        (typically 32 bits on a 32-bit machine and 64 bits on a 64-bit machine,
        but it varies with the OS and compiler). The exact set of functions that
        $(STD_DATE) provides for using with $(D d_time) aren't the same as what
        C provides for using with $(D time_t), but their representation of time
        is virtually the same.)

    $(P $(STD_DATETIME), on the other hand, is very much an object-oriented
        solution, and it's not C-based at all. Rather, its API is based on
        $(HTTP www.boost.org/doc/libs/release/doc/html/date_time.html, Boost)'s
        types for handling dates and times (though they're far from identical).
        So, it's a bit of a paradigm shift to move from $(STD_DATE) to
        $(STD_DATETIME). Don't expect much to be the same between the two.
        However, $(STD_DATETIME) is not plagued with the same bugs that
        $(STD_DATE) is plagued with ($(STD_DATE) being quite buggy in general),
        and it provides much more functionality than $(STD_DATE) does. So, in
        the long run at least, dealing with $(STD_DATETIME) should be much more
        pleasant - though migration is likely to present a bit of a hurdle in
        the short term.)

    $(H2 Basic Concepts of std.datetime)

    $(P Most things in $(STD_DATETIME) are based on and/or use three concepts:)

    $(UL
        $(LI $(B Duration): A duration of time with units. e.g. 4 days or 72
             seconds.)
        $(LI $(B Time Point): A specific point in time. e.g. 21:00 or May 7th,
             2013.)
        $(LI $(B Time Interval): A fixed period of time. e.g. [02:44 -
             12:22$(RPAREN) or [February 4th, 1922 - January 17th,
             1955$(RPAREN).)
     )

    $(H3 Durations)

    $(P The duration types can actually be found in $(CORE_TIME). They are
        $(DURATION) and $(TICK_DURATION). $(TICK_DURATION) is intended for
        precision timing and is used primarily with $(XREF datetime, StopWatch)
        and the benchmarking functions found in $(STD_DATETIME) - such as
        $(XREF datetime, benchmark) - and you're unlikely to use it outside of
        using them. $(DURATION), on the other hand, you're likely to use quite a
        bit.)

    $(P $(DURATION) holds its time internally as hecto-nanoseconds (100 ns), so
        that's its maximum precision. It has property functions for both
        returning the duration of time truncated to a particular unit (such as
        the $(D days) and $(D seconds) property functions) as well as a function
        for returning the total number of a particular unit in that $(DURATION)
        (the $(D total) function).)

    $(P Generally, a $(DURATION) is created in one of two ways: by subtracting
        two time points or with the $(FULL_CXREF time, dur) function. So, for
        instance, if you subtracted a time point which represented 17:02 from
        a time point which represented 6:07, you'd get a $(DURATION) which
        represented 10 hours and 55 minutes. Or, if you wanted to create a
        $(DURATION) directly, then you'd use the $(D dur) function and make
        a call like $(D dur!"hours"(17)) or $(D dur!"seconds"(234)).)

--------------------
auto duration = TimeOfDay(17, 2) - TimeOfDay(6, 7);
assert(duration == dur!"hours"(10) + dur!"minutes"(55));
assert(duration.hours == 10);
assert(duration.minutes == 55);
assert(duration.total!"hours"() == 10);
assert(duration.total!"minutes"() == 655);
assert(duration.total!"hnsecs"() == 393_000_000_000);
--------------------

    $(P Like any number, $(DURATION)s can be added together or
        subtracted from. However, unlike a naked number, they have units
        associated with them and will handle the appropriate conversions.
        Also, it should be noted that the various functions in druntime and
        Phobos which take a duration of time take an actual $(DURATION) rather
        than a naked number (most currently take both, though the versions which
        take a naked number are going to be deprecated). For instance,
        $(FULL_CXREF thread, sleep) takes a $(DURATION), as does
        $(FULL_XREF concurrency, receiveTimeout). So, durations are used outside
        of just interacting with $(CORE_TIME) and $(STD_DATETIME).)

    $(P One particular thing to note here is how both $(D dur) and $(D total)
        take a string representing the units of time to be used. This is an
        idiom used throughout $(CORE_TIME) and $(STD_DATETIME). The possible
        units are $(D "years"), $(D "months"), $(D "weeks"), $(D "days"),
        $(D "hours"), $(D "minutes"), $(D "seconds"), $(D "msecs"),
        $(D "usecs"), $(D "hnsecs"), and $(D "nsecs"). It should be noted
        however that very few functions take $(D "nsecs"), because nothing in
        $(STD_DATETIME), and very little in $(CORE_TIME), has precision
        greater than hnsecs (100 ns). Also, a number of functions (such as
        $(FULL_CXREF time, dur)) do not take $(D "years") or $(D "months"),
        because it is not possible to convert between years or months and
        smaller units without a specific date. So, while you can add a
        $(DURATION) to a time point, if you want to add years or months to one,
        you must use a separate function (such as $(D add)) to do that - and
        those $(I will) take $(D "years") and $(D "months").)

    $(H3 Time Points)

    $(P $(STD_DATETIME) has 4 types which represent time points.)

    $(UL
        $(LI $(DATE))
        $(LI $(TIMEOFDAY))
        $(LI $(DATETIME))
        $(LI $(SYSTIME))
     )

    $(P A $(DATE) represents a date and holds its year, month, and day as
        separate values internally. A $(TIMEOFDAY) represents a time of day,
        00:00:00 - 23:59:59, and holds its hour, minute, and second as separate
        values internally. A $(DATETIME) represents a date and time and holds
        its values as a $(DATE) and $(TIMEOFDAY) internally. None of these types
        have any concept of time zone. They represent generic dates and/or times
        and are best-suited for cases where you need a date and/or time but
        don't care about time zone. Also, because they hold their values
        separated internally, those values don't have to be calculated every
        time that you ask for them.)

--------------------
auto date = Date(1992, 12, 27);
auto tod = TimeOfDay(7, 0, 22);
auto dateTime = DateTime(1992, 12, 27, 7, 0, 22);
assert(date == dateTime.date);
assert(tod == dateTime.timeOfDay);
--------------------

    $(P A $(SYSTIME), however, is an entirely different beast. It represents
        a date and time - similar to $(DATETIME) - but it goes to hnsec
        precision instead of only second precision, and it incorporates the
        concept of time zone. Its time is held internally as a 64-bit integral
        value which holds the number of hnsecs which have passed since midnight,
        January 1st, 1 A.D. in UTC. It also has a $(TIMEZONE) object which it
        uses to polymorphically adjust its UTC value to the appropriate time
        zone when querying for values such as its year or hour.)

    $(P $(SYSTIME) is the type which is used to interface with the system's
        clock. When you ask for the current time, you get a $(SYSTIME). And
        because it always holds its internal value in UTC, it never has problems
        with DST or time zone changes. It has most of the functions that
        $(DATETIME) has as well as a number of functions specific to it. It can
        be cast to the other 3 time point types as well as be constructed from
        them, but you do risk problems with DST when creating a $(SYSTIME) from
        the other 3 time points unless you specifically create the $(SYSTIME)
        with a $(TIMEZONE) which doesn't have DST (such as
        $(FULL_XREF datetime, UTC)), since when a time zone has DST, one hour of
        the year does not exist, and another exists twice. You can also convert
        to and from unix time, which is what you're dealing with in C with
        $(D time_t).)

    $(P The one other related type which I should mention at this point is
        $(FULL_CXREF time, FracSec). It holds fractional seconds, and it is
        what you get from a $(DURATION) or $(SYSTIME) when you specifically
        ask for the fractional portion of the time.)

--------------------
auto st1 = Clock.currTime(); //Current time in local time.
auto st2 = Clock.currTime(UTC()); //Current time in UTC.
auto st3 = SysTime(DateTime(1992, 12, 27, 7, 0, 22), FracSec.from!"usecs"(5));
assert((cast(Date)st3) == Date(1992, 12, 27));
assert((cast(TimeOfDay)st3) == TimeOfDay(7, 0, 22));
assert((cast(DateTime)st3) == DateTime(1992, 12, 27, 7, 0, 22));
assert(st3.fracSec == FracSec.from!"hnsecs"(50));
--------------------

    $(H3 Time Intervals)

    $(P $(STD_DATE) has nothing to correspond to time intervals, so I won't
        go over them in great detail. Essentially, they're constructed from
        either two time points or a time point and a duration. $(INTERVAL)
        is a finite time interval with two end points, whereas
        $(POSINF_INTERVAL) is an infinite time interval starting at a specific
        time point and going to positive infinity, and $(NEGINF_INTERVAL)
        is an infinite time interval starting at negative infinity and going
        to a specific time point. They have various operations for dealing
        with intersections and the like. It is also possible to create ranges
        over them if you want to operate on a range of time points. Take a
        look at the documentation for more details.)

    $(H2 Interfacing with C)

    $(P Hopefully, you can do everything that you need to do using the types in
        $(CORE_TIME) and $(STD_DATETIME), but if you do need to interface with C
        code, then you can. C's $(D time_t) uses "unix time" (seconds since
        midnight, January 1st, 1970 A.D. in UTC), whereas $(SYSTIME) uses what
        it calls "std time" (hnsecs since midnight January 1st, 1 A.D. in UTC).
        Translating between the two is fairly straightforward. To get a
        $(D time_t) from a $(SYSTIME), simply call $(D toUnixTime) on the
        $(SYSTIME). To convert the other way around, you first need to convert
        a $(D time_t) to std time, then pass that value to $(SYSTIME)'s
        constructor. And if you ever simply need a $(SYSTIME)'s std time for
        any reason, then use its $(D stdTime) property.)

--------------------
time_t unixTime = core.stdc.time.time(null);
auto stdTime = unixTimeToStdTime(unixTime);
auto st = SysTime(stdTime);
assert(unixTime == st.toUnixTime());
assert(stdTime == st.stdTime);
--------------------

    $(P Hecto-nanoseconds were chosen as the internal representation of
        $(DURATION) and $(SYSTIME), because that is the highest precision that
        you can use with a 64-bit integer and still cover a reasonable amount
        of time ($(SYSTIME) covers from around 29,000 B.C. to around 29,000
        A.D.). It also happens to be the same internal representation that
        C# uses, so if you need to interface with C# for any reason, converting
        between its representation of time and $(STD_DATETIME)'s representation
        is extremely easy, since no conversion is necessary.
        $(HTTP msdn.microsoft.com/en-us/library/system.datetime.aspx,
        C#'s $(D DateTime)) uses both the same units and epoch for its internal
        representation (which it calls $(D Ticks)) as $(SYSTIME), though unlike
        $(SYSTIME), it doesn't work with negative values (which would be B.C.)
        and doesn't go past the end of 9,999 A.D. Most programs are unlikely to
        care about values outside that range however. Regardless, hnsecs make
        the most sense for $(STD_DATETIME), which tries to have the highest
        precision that it reasonably can, so that's why they were picked.)

    $(H2 Recommendations on Using std.datetime)

    $(P Whether $(DATE), $(TIMEOFDAY), $(DATETIME), or $(SYSTIME) is more
        appropriate in a particular situation depends very much on that
        situation. $(DATE), $(TIMEOFDAY), and $(DATETIME) generally make the
        most sense when you're dealing with generic dates and times that have
        nothing to do with time zones, but if you're dealing with time zones
        at all or are dealing with anything which needs to worry about DST,
        you should use $(SYSTIME). Because it keeps its time internally in UTC,
        it avoids problems with DST. And while it does have a time zone
        component, it defaults to $(FULL_XREF datetime, LocalTime) (which
        is the time zone type for the local time of the system), so you don't
        generally have to deal directly with time zones if you don't want to.)

    $(P If you $(I do) want to deal with time zones, then the time zone types
        in $(STD_DATETIME) are $(LOCALTIME), $(UTC), $(SIMPLETZ), $(POSIXTZ),
        and $(WINDOWSTZ) - or if for some reason, they don't do what you need,
        you can always create your own time zone class derived from $(TIMEZONE).
        That's unlikely to be necessary, however (and if you think that you
        have come up with such a class which would be generally useful, please
        bring it up in the
        $(HTTP www.digitalmars.com/NewsGroup.html, digitalmars.D newsgroup),
        since if it's truly generally useful, we may want some version of it in
        $(STD_DATETIME)). Read their documentation for more details. Most
        applications shouldn't have to worry about time zones though, beyond
        perhaps using $(UTC) instead of $(LOCALTIME) in some cases.)

    $(P When it comes to saving a time point to disk or a database or something
        similar, I would generally recommend using the $(D toISOString) or
        $(D toISOExtString) functions, since both are standard ISO formats
        for date-time strings ($(D toISOExtString) is likely better in the
        general case, since it's more humanly readable, but they're both
        standard). You can then use $(D fromISOString) or $(D fromISOExtString)
        to recreate the appropriate time type later. $(D toString) uses the
        $(D toSimpleString) function, which is an invention of Boost and is
        somewhat more humanly readable, but it isn't standard, so you probably
        shouldn't use it for saving time point values.)

--------------------
auto dateTime = DateTime(1997, 5, 4, 12, 22, 3);
assert(dateTime.toISOString() == "19970504T122203");
assert(dateTime.toISOExtString() == "1997-05-04T12:22:03");
assert(dateTime.toSimpleString() == "1997-May-04 12:22:03");
auto restored = DateTime.fromISOExtString(dateTime.toISOExtString());
assert(dateTime == restored);
--------------------

    $(P One area with saving times as strings which gets a bit awkward is time
        zones. The time zone is included in the string as part of the ISO
        standard, but all it contains is the total offset from UTC at that
        particular date and time, so you can't generally use an ISO string
        (extended or otherwise) to get the exact time zone which the $(SYSTIME)
        originally had. Rather, it will be restored with a $(SIMPLETZ) with the
        given offset from UTC (except in the case of UTC, where it can restore
        $(UTC)). On the other hand, if you're using $(LOCALTIME), then the time
        zone is not part of the string (per the ISO standard), and restoring the
        $(SYSTIME) will restore it to whatever the current time zone is on the
        box, regardless of what the original time zone was. However, because in
        all cases, except for $(LOCALTIME), the UTC offset is included in the
        string, it is generally possible to get the exact UTC time that the
        $(SYSTIME) was for. But you can't usually restore the original time zone
        from just the ISO string.)

--------------------
auto local = SysTime(629_983_705_230_000_035);
auto utc = local.toUTC();
auto other = local.toOtherTZ(TimeZone.getTimeZone("America/New_York"));

//This assumes that you're in "America/Los_Angeles". You'd get a different
//time if you're in a different time zone.
assert(local.toISOExtString() == "1997-05-04T12:22:03.0000035");

assert(utc.toISOExtString() == "1997-05-04T19:22:03.0000035Z");
assert(other.toISOExtString() == "1997-05-04T15:22:03.0000035-04:00");

auto restLocal = SysTime.fromISOExtString(local.toISOExtString());
auto restUTC = SysTime.fromISOExtString(utc.toISOExtString());
auto restOther = SysTime.fromISOExtString(other.toISOExtString());

//Only guaranteed because it's on the same machine.
assert(restLocal == local);

//Guaranteed regardless of machine. Their internal values could differ however.
assert(cast(DateTime)restLocal == cast(DateTime)local);

//Time zone is UTC for both.
assert(restUTC == utc);

//Time zone for restOther is SimpleTimeZone(-4 * 60), not "America/New_York".
assert(restOther == other);
--------------------

    $(P To summarize, $(UTC) and $(SIMPLETZ) can be restored exactly using an
        ISO or ISO extended string. However, none of the other $(TIMEZONE)s can
        be. $(LOCALTIME) is restored with the same date and time but in the
        local time zone of the computer it's restored on, so its std time may
        differ. Other time zones end up with the restored $(SYSTIME) having the
        same std time as the original, but the new time zone is a $(SIMPLETZ)
        with the same total UTC offset which the original time zone had at the
        given std time, but you don't get the original time zone back. That
        works just fine if you don't ever need to change the value of that
        $(SYSTIME) or need to know the name of the original time zone, but it is
        inadequate if you need to do either of those, since the rules for
        the new time zone won't match those of the original.)

    $(P So, if you don't care about the time zone or if the restored $(SYSTIME)
        has the same std time as the original, then $(LOCALTIME) is fine.
        However, if you want the std time to be consistent, then avoid
        $(LOCALTIME). In most cases, I'd advise simply using $(UTC) as the
        time zone when saving the time. And if you want to restore the time zone
        such that it's the same time zone with the same rules as it was prior to
        saving the time, then you're going to need to save that information
        yourself. With a $(POSIXTZ) or a $(WINDOWSTZ), all you have to do is
        save the time zone's $(D name) (which for them is the TZ database name
        and the Windows time zone name of that time zone respectively). That can
        be used to restore the time zone. If it's a $(SIMPLETZ) or $(UTC), then
        you don't have to do anything, because the ISO string will be enough. If
        you're using $(LOCALTIME), however, you're in a bit of a bind.)

    $(P The restored time zone will be $(LOCALTIME), so if you want it to be
        whatever the local time of the computer you're doing the restoring on
        is, then you're fine. But if you want to be able to have the same actual
        time zone restored regardless of the local time of the computer
        restoring the time, you'll need to figure out what the time zone's TZ
        database name or Windows time zone name is on the original computer so
        that you can use it to get its corresponding $(POSIXTZ) or $(WINDOWSTZ)
        on the computer that's doing the restoring. But it's actually
        $(I really) hard to accurately determine the TZ database name or Windows
        time zone name of the local time zone on any OS other than on Windows,
        so $(STD_DATETIME) doesn't currently provide a way to do that. I expect
        that such a requirement would be quite rare however. In most cases,
        you'll care about $(LOCALTIME) and/or $(UTC), and even if you're using
        $(POSIXTZ) or $(WINDOWSTZ), odds are that restoring the time with the
        correct std time value and correct UTC offset will be enough (and if
        it's not, you can always save the time zone's name to restore the
        correct $(POSIXTZ) or $(WINDOWSTZ)). It's just $(LOCALTIME) that has
        the problem. However, if a function to accurately determine the TZ
        database name of the local time zone on Posix systems is ever devised,
        then it will be added to $(STD_DATETIME).)

    $(P The other, more compact, option for saving a $(SYSTIME) is to just save
        its std time as a 64-bit integer. It's not really humanly readable
        like an ISO or ISO extended string would be, but it takes up less space
        if saved as an actual number rather than a string. However, you do then
        have to worry about the time zone yourself entirely if you wish to be
        able to restore it. But if you save the current UTC offset (meaning
        the UTC offset with the DST offset applied - such as an ISO string would
        include), that would be enough to correctly give what the time would
        have been in the original time zone even if you can't restore that time
        zone.)

    $(P Well, that's probably more than enough on time zones. In most cases, you
        shouldn't need to care about them ($(SYSTIME) is designed to make it
        so that you shouldn't have to worry about them if you don't want to),
        but $(STD_DATETIME) strives to give the best tools possible for handling
        time zones when you actually want to. Regardless, by far the biggest
        gain that $(SYSTIME) gives you is that its internal time is always in
        UTC, so regardless of whether you try and do anything with time zones
        explicitly, you won't have any problems with DST changes when dealing
        with $(SYSTIME).)

    $(P One last suggestion on using $(SYSTIME) would be that if you need to
        query it for more than one of its properties (e.g. $(D day) or
        $(D hour)), or if you need to do it many times in a row, and the
        $(SYSTIME) isn't going to change, then you should probably cast it to
        another time point type (probably $(DATETIME)) and query $(I it) for
        those values. The reason for this is that every time that you call a
        property function on a $(SYSTIME), it has to convert its internal std
        time to the value of the property that you're asking for, whereas if you
        convert it to a $(DATETIME), the $(DATETIME) holds those values
        separately, and you only have to do the calculations once - when you
        do the conversion from the $(SYSTIME) to a $(DATETIME). If what you're
        doing doesn't need that extra boost of efficiency, then you might as
        well not bother, but it is good to be aware that it's less efficient to
        query each of $(SYSTIME)'s properties individually rather than
        converting it to a $(DATETIME) and then querying it.)

--------------------
auto st = Clock.currTime();

//Each value must be individually calculated.
{
    auto year = st.year;
    auto month = st.month;
    auto day = st.day;
    auto hour = st.hour;
    auto minute = st.minute;
    auto second = st.second;
    auto fracSec = st.fracSec;
}

/+
    You do the calculations only twice if you convert to a DateTime
    (twice instead of once, because you're still asking for FracSec
    separately, though that particular calculation is fairly cheap).
 +/
auto dateTime = cast(DateTime)st;
{
    auto year = dateTime.year;
    auto month = dateTime.month;
    auto day = dateTime.day;
    auto hour = dateTime.hour;
    auto minute = dateTime.minute;
    auto second = dateTime.second;
    auto fracSec = st.fracSec;
}
--------------------

    $(H2 Migrating to std.datetime)

    $(P Okay, hopefully you have a fair idea of the basics of $(STD_DATETIME)
        at this point (though there's plenty more which is covered in the
        documentation), but the big question for many is how best to handle
        converting your code from using $(STD_DATE) to using $(STD_DATETIME).
        When using $(STD_DATE), you would have been using $(D d_time), and if
        you had to worry about time zones, you were probably either using C
        functions to deal with conversions or just doing them yourself, since
        the functionality in $(STD_DATE) which relates to time zones is rather
        broken. As a result, most of what you would have done would likely be in
        UTC.)

    $(P $(SYSTIME) holds its time internally in UTC in a manner similar to
        $(D d_time) (albeit with different units and a different epoch), and it
        is the type intended for dealing with the time from the system's clock,
        so $(SYSTIME) is generally what $(D d_time) should be replaced with.
        Functions in Phobos which previously took or returned a $(D d_time) now
        take or return a $(SYSTIME) or are scheduled to be deprecated and have
        replacement functions which take or return a $(SYSTIME). Generally, in
        the case where a function took a $(D d_time), that function is now
        overloaded with a version which takes a $(SYSTIME), but in cases where a
        function could not be overloaded (such as when it simply returned a
        $(D d_time)), a new function has been added to replace the old one (so
        as to avoid breaking existing code). The module that this impacts the
        most is $(STD_FILE). For example,)

--------------------
d_time dTime = "myfile.txt".lastModified();
SysTime sysTime = "myfile.txt".timeLastModified();

setTimes("yourfile.txt", dTime, dTime + 5);
setTimes("yourfile.txt", sysTime, sysTime + dur!"msecs"(5));
--------------------

    $(P With all such functions, it's simply a matter of changing the type of
        the argument that you're passing to the function or assigning its
        return value to and possibly changing the function name that you're
        calling so that it's the version that returns a $(SYSTIME). Those
        changes are quite straightforward and not particularly disruptive. Of
        greater concern are the formats that times are printed or saved in
        and how time zones are dealt with.)

    $(P If you were saving the integral $(D d_time) value anywhere, then you're
        either going to have to switch to saving a value that $(SYSTIME) would
        use as discussed previously (such as its std time or its ISO string),
        or you're going to have be converting between $(D d_time) and
        $(SYSTIME). At present, the functions
        $(FULL_XREF datetime, sysTimeToDTime) and
        $(FULL_XREF datetime, dTimeToSysTime) will do those conversions for you.
        So, converting between the two formats is easy. However, because
        $(D d_time) is going away, those functions will be going away. That
        means that you either need to refactor your code so that those functions
        aren't necessary, or you need to copy them to your own code to continue
        to use them.)

     $(P As for formatted strings, $(STD_DATETIME) currently only supports
         ISO strings, ISO extended strings, and Boost's simple string.
         Eventually, it should have functions for custom strings, but a
         well-designed function for creating custom strings based on format
         strings is not easy to design, and it hasn't been done for
         $(STD_DATETIME) yet (it's on my todo list, but it could be a while
         before I get to it). So, in general, you're either going to have to
         switch to using one of the string formats that $(STD_DATETIME)
         supports, or you're going to have to generate and parse the string
         format that you want yourself. In some cases, you should be able to
         adjust the string that $(D core.stdc.time.ctime) gives you, and in
         others, you may be able to use $(D toISOExtString) and adjust what
         $(I it) gives you, but there's a decent chance that you're going to
         have to just create and parse the strings yourself using the various
         properties on $(SYSTIME). One major difference between the string
         functions in $(STD_DATE) and those in $(STD_DATETIME) to note is that
         unlike $(STD_DATE), aside from Boost's simple strings, nothing in
         $(STD_DATETIME) prints the names of months or weekdays, because that
         poses a localization issue. So, unless you're using
         $(D core.stdc.time.ctime) to get those values, you're going to have
         to create the names yourself.)

     $(P Now, if you were doing anything with time zones with $(STD_DATE), odds
         are that you were doing all of those conversions yourself (since that's
         one of the areas where $(STD_DATE) is buggy). That being the case,
         you probably have the offset from UTC and the offset adjustment for
         DST for whatever time zone you're dealing with. What is likely the
         best way to handle that is to create a $(SIMPLETZ) using those values.
         Simply calculate the total UTC offset (so add in the DST offset if
         it applies for the date in question) in minutes and create a $(SIMPLETZ)
         with that. Note that $(STD_DATETIME) treats west of UTC as
         $(I negative) (for some reason, some systems - particularly Posix stuff
         - use a positive offset from UTC when west of UTC, in spite of the fact
         that when talking about time zones, negative is always used for west of
         UTC, and that's what the ISO standard strings do). So, you may have to
         adjust your values accordingly. Regardless, be very careful to make
         sure that you understand what the values you've been using represent
         in units of time and whether you need to be adding or subtracting them
         to convert them to what $(SIMPLETZ) expects for its offset from UTC:
         the minutes to $(I add) to the time in UTC to get the time in the
         target time zone.)

--------------------
//These are the same offsets as America/Los_Angeles.
auto utcOffset = -8 * 60;
auto dstOffset = 60;

immutable tzWithDST = new SimpleTimeZone(utcOffset + dstOffset);
immutable tzWithoutDST = new SimpleTimeZone(utcOffset);
--------------------

     $(P The last thing that I have to note is some differences in numerical
         values between $(STD_DATE) and $(STD_DATETIME).
         $(FULL_XREF date, Date)'s $(D weekday) property gives Sunday a value of
         1, but $(FULL_XREF date, weekDay) gives Sunday a value of 0.
         $(FULL_XREF datetime, DayOfWeek) gives Sunday a value of 0. So,
         depending on which part of $(STD_DATE) you're dealing with it, it
         may or may not match what $(STD_DATETIME) is doing for the numerical
         values of weekdays. Months have a similar problem.
         $(FULL_XREF date, Date)'s $(D month) property gives January a value of
         1 - which matches what $(FULL_XREF datetime, Month) does - but
         $(FULL_XREF date, monthFromTime) gives January a value of 0. So,
         just as with the days of the week, you have to be careful with the
         numerical values of the months. Whether $(STD_DATETIME) matches what
         $(STD_DATE) is doing depends on which part of $(STD_DATE) you're using.
         And as you'll notice, it's not even consistent as to whether
         $(FULL_XREF date, Date) or the free function in $(STD_DATE) is the one
         which matches $(STD_DATETIME). So, you should be very careful when
         converting code which uses numerical values for either the days of the
         week or the months of the year.)

    $(BOOKTABLE $(H3 std.date symbols and their std.datetime counterparts),
        $(TR $(TD $(B $(STD_DATE)))
             $(TD $(B $(STD_DATETIME) Equivalent)))
        $(TR $(TD $(XREF date, d_time))
             $(TD The closest would be $(SYSTIME).))
        $(TR $(TD $(XREF date, d_time_nan))
             $(TD There is no equivalent. $(SYSTIME).$(D init), which has a
                  $(D null) $(TIMEZONE) object, would be the closest, but once
                  CTFE advances to the point that you can new up class objects
                  with it, $(SYSTIME).$(D init)'s $(D timezone) will be
                  $(LOCALTIME), so don't rely on $(SYSTIME).$(D init) being
                  invalid. $(STD_DATETIME) in general tries to avoid having any
                  invalid states for any of its types. It's intended that
                  creating such values be impossible.))
        $(TR $(TD $(XREF date, Date))
             $(TD $(SYSTIME)))
        $(TR $(TD $(XREF date, Date).$(D year))
             $(TD $(SYSTIME).$(D year)))
        $(TR $(TD $(XREF date, Date).$(D month))
             $(TD $(SYSTIME).$(D month)))
        $(TR $(TD $(XREF date, Date).$(D day))
             $(TD $(SYSTIME).$(D day)))
        $(TR $(TD $(XREF date, Date).$(D hour))
             $(TD $(SYSTIME).$(D hour)))
        $(TR $(TD $(XREF date, Date).$(D minute))
             $(TD $(SYSTIME).$(D minute)))
        $(TR $(TD $(XREF date, Date).$(D second))
             $(TD $(SYSTIME).$(D second)))
        $(TR $(TD $(XREF date, Date).$(D ms))
             $(TD $(SYSTIME).$(D fracSec.msecs)))
        $(TR $(TD $(XREF date, Date).$(D weekday))
             $(TD $(SYSTIME).$(D dayOfWeek) - but note that the values are off
                  by 1.))
        $(TR $(TD $(XREF date, Date).$(D tzcorrection))
             $(TD
--------------------
immutable tz = sysTime.timezone;
auto diff = tz.utcToTZ(sysTime.stdTime) - sysTime.stdTime;
auto tzcorrection = convert!("hnsecs", "minutes")(diff);
--------------------
However, it looks like $(D tzcorrection) is broken, so you're probably not using
it in your code anyway.
              ))
        $(TR $(TD $(XREF date, Date).$(D parse))
             $(TD $(SYSTIME).$(D fromISOString),
                  $(SYSTIME).$(D fromISOExtString), and
                  $(SYSTIME).$(D fromSimpleString), but the formats of the
                  strings differ from what $(FULL_XREF date, Date).$(D parse)
                  accepts.))
        $(TR $(TD $(XREF date, ticksPerSecond))
             $(TD There is no equivalent. It's only relevant to $(D d_time).))
        $(TR $(TD $(XREF date, toISO8601YearWeek))
             $(TD $(SYSTIME).$(D isoWeek)))
        $(TR $(TD $(XREF date, hourFromTime))
             $(TD $(SYSTIME).$(D hour)))
        $(TR $(TD $(XREF date, minFromTime))
             $(TD $(SYSTIME).$(D minute)))
        $(TR $(TD $(XREF date, secFromTime))
             $(TD $(SYSTIME).$(D second)))
        $(TR $(TD $(XREF date, daysInYear))
             $(TD $(D sysTime.isLeapYear ? 366 : 365)))
        $(TR $(TD $(XREF date, dayFromYear))
             $(TD $(D (sysTime - SysTime(Date(1970, 1, 1), UTC())).total!"days"())))
        $(TR $(TD $(XREF date, yearFromTime))
             $(TD $(SYSTIME).$(D year)))
        $(TR $(TD $(XREF date, inLeapYear))
             $(TD $(SYSTIME).$(D isLeapYear)))
        $(TR $(TD $(XREF date, monthFromTime))
             $(TD $(SYSTIME).$(D month) - but note that the values are off by
                  1.))
        $(TR $(TD $(XREF date, dateFromTime))
             $(TD $(SYSTIME).$(D day)))
        $(TR $(TD $(XREF date, weekDay))
             $(TD $(SYSTIME).$(D dayOfWeek)))
        $(TR $(TD $(XREF date, UTCtoLocalTime))
             $(TD $(SYSTIME).$(D toUTC)))
        $(TR $(TD $(XREF date, dateFromNthWeekdayOfMonth))
             $(TD
--------------------
//There is no equivalent. This is a possible implementation.
int dateFromNthWeekdayOfMonth(int year, Month month,
                              DayOfWeek dow, int n)
{
    auto first = Date(year, month, 1);
    auto target = first;
    immutable targetDOTW = target.dayOfWeek;

    if (targetDOTW != dow)
    {
        if (targetDOTW < dow)
            target += dur!"days"(dow - targetDOTW);
        else
        {
            target += dur!"days"((DayOfWeek.sat - targetDOTW) +
                                 dow + 1);
        }
    }

    target += dur!"weeks"(n - 1);

    if (target.month != first.month)
        target -= dur!"weeks"(1);

    return cast(int)((target - first).total!"days"()) + 1;
}
--------------------
              ))
        $(TR $(TD $(XREF date, daysInMonth))
             $(TD $(SYSTIME).$(D endOfMonthDay); Actually, this name is overly
                  easy to confuse with $(D endOfMonth) - which returns a
                  $(SYSTIME) of the last day of the month. I will probably
                  rename this to $(D daysInMonth). But if I do, it won't be
                  until the next release (2.054), and this name will be around
                  until it's gone through the full deprecation cycle.))
        $(TR $(TD $(XREF date, UTCtoString))
             $(TD There is no equivalent. You could probably parse and recombine
                  $(D core.stdc.time.ctime) and
                  $(SYSTIME).$(D toISOExtString) to create it though. However,
                  this function appears to be fairly buggy in the first place,
                  so odds are that your code isn't using it anyway.))
        $(TR $(TD $(XREF date, toUTCString))
             $(TD There is no equivalent. You could probably parse and recombine
                  $(D core.stdc.time.ctime) and
                  $(SYSTIME).$(D toISOExtString) to create it though.))
        $(TR $(TD $(XREF date, toDateString))
             $(TD There is no equivalent. You could probably parse and recombine
                  $(D core.stdc.time.ctime) and
                  $(SYSTIME).$(D toISOExtString) to create it though. However,
                  this function appears to be fairly buggy in the first place,
                  so odds are that your code isn't using it anyway.))
        $(TR $(TD $(XREF date, toTimeString))
             $(TD There is no equivalent. You could probably parse and recombine
                  $(D core.stdc.time.ctime) and
                  $(SYSTIME).$(D toISOExtString) to create it though. However,
                  this function appears to be fairly buggy in the first place,
                  so odds are that your code isn't using it anyway.))
        $(TR $(TD $(XREF date, parse).$(D parse))
             $(TD $(SYSTIME).$(D fromISOString),
                  $(SYSTIME).$(D fromISOExtString), and
                  $(SYSTIME).$(D fromSimpleString), but the formats of the
                  strings differ from what $(FULL_XREF date, parse) accepts.))
        $(TR $(TD $(XREF date, getUTCtime))
             $(TD $(D Clock.currTime(UTC())) if you want the $(SYSTIME) to have
                  its time zone be $(UTC). More likely though, you'll just use
                  $(D Clock.currTime()). Its internal time is in UTC
                  regardless.))
        $(TR $(TD $(XREF date, DosFileTime))
             $(TD $(XREF datetime, DosFileTime)))
        $(TR $(TD $(XREF date, toDtime))
             $(TD $(XREF datetime, DosFileTimeToSysTime)))
        $(TR $(TD $(XREF date, toDosFileTime))
             $(TD $(XREF datetime, SysTimeToDosFileTime)))
        $(TR $(TD $(XREF date, benchmark))
             $(TD $(XREF datetime, benchmark)))
    )

    $(P Note that I'm not an expert on what does and doesn't work in
        $(STD_DATE), so while I have noted some of the functions that I know to
        be broken, just because a function isn't labeled as broken in the above
        table does not mean that it works correctly. And any function which
        doesn't work correctly is obviously not going to give the same results
        as the $(STD_DATETIME) equivalent, since it's almost certain that the
        $(STD_DATETIME) version isn't buggy, let alone buggy in the same way (if
        it is buggy, the bug is almost certainly going to be far more subtle
        than any bug in $(STD_DATE), since $(STD_DATETIME) is quite thoroughly
        unit tested).)

    $(H2 Conclusion)

    $(P Hopefully this article has improved your understanding of
        $(STD_DATETIME) and will get you well on your way to being able to
        migrate your code from $(STD_DATE) to $(STD_DATETIME). If you have any
        further questions, please ask them on the
        $(HTTP www.digitalmars.com/NewsGroup.html, digitalmars.D.learn newsgroup).
        And if there's a major use case of $(STD_DATE) which is not easy to
        convert over to $(STD_DATETIME) which I missed in this
        article and you think should be in it, please feel free to bring it up
        on the
        $(HTTP www.digitalmars.com/NewsGroup.html, digitalmars.D newsgroup),
        and if need be, I'll update this article with the relevant information.)
 )

Macros:
    CATEGORY_ARTICLES=$0
    BOOKTABLE = <center><table cellspacing=0 cellpadding=5 valign=top class="book"><caption>$1</caption>$2</table></center>
    RED = <span style="color:red">$0</span>
    TITLE = Introduction to std.datetime

    CXREF = <a href="phobos/core_$1.html#$2">$(D $2)</a>
    FULL_CXREF = <a href="phobos/core_$1.html#$2">$(D core.$1.$2)</a>
    XREF = <a href="phobos/std_$1.html#$2">$(D $2)</a>
    CORE_TIME=<a href="phobos/core_time.html">$(D core.time)</a>
    STD_DATETIME=<a href="phobos/std_datetime.html">$(D std.datetime)</a>
    STD_DATE=<a href="phobos/std_date.html">$(D std.date)</a>
    STD_FILE=<a href="phobos/std_file.html">$(D std.file)</a>

    DATE=$(XREF datetime, Date)
    DATETIME=$(XREF datetime, DateTime)
    DURATION=$(CXREF time, Duration)
    FRACSEC=$(CXREF time, FracSec)
    INTERVAL=$(XREF datetime, Interval)
    LOCALTIME=$(XREF datetime, LocalTime)
    NEGINF_INTERVAL=$(XREF datetime, NegInfInterval)
    POSINF_INTERVAL=$(XREF datetime, PosInfInterval)
    POSIXTZ=$(XREF datetime, PosixTimeZone)
    STOPWATCH=$(XREF datetime, StopWatch)
    SIMPLETZ=$(XREF datetime, SimpleTimeZone)
    SYSTIME=$(XREF datetime, SysTime)
    TIMEOFDAY=$(XREF datetime, TimeOfDay)
    TICK_DURATION=$(CXREF time, TickDuration)
    TIMEZONE=$(XREF datetime, TimeZone)
    UTC=$(XREF datetime, UTC)
    WINDOWSTZ=$(XREF datetime, WindowsTimeZone)
    WIKI=
    _=