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Erlang date, time, and timezone management: formatting, conversion, and date arithmetic
Erlang
branch: master

README.markdown

qdate - Erlang Date and Timezone Library

Build Status

Purpose

Erlang Date and Time management is rather primitive, but improving.

dh_date, of which ec_date in erlware_commons is a fork, is a huge step towards formatting and parsing dates in a way that compares nicely with PHP's date and strtotime functions.

Unfortunately, ec_date doesn't deal with timezones, but conveniently, the project erlang_localtime does.

It is the express purpose of this qdate package to bring together the benefits of ec_date and erlang_localtime, as well as extending the capabilities of both to provide for other needed tools found in a single module.

qdate provides date and time formatting and parsing from and into:

  • Formatting Strings
  • Erlang Date Format
  • Erlang Now Format
  • Unixtime integers
  • Timezones

And all this while dealing with timezone parsing, formatting, conversion and overall management.

Acceptable Date Formats

  • Erlang Date Format: {{Y,M,D},{H,M,S}}
  • Erlang Now Format: {MegaSecs, Secs, MicroSecs}
  • Date String: "2013-12-31 08:15pm" (including custom formats as defined with qdate:register_parser/2 - see below)
  • Integer Unix Timestamp: 1388448000
  • A Two-tuple, where the first element is one of the above, and the second is a timezone. (i.e. {{{2008,12,21},{23,59,45}}, "EST"} or {"2008-12-21 11:59:45pm", "EST"}). Note: While, you can specify a timezone along with unix timestamps or the Erlang now format, it won't do anything, as both of those timestamps are absolute, and imply GMT.

All while doing so by allowing you to either set a timezone by some arbitrary key or by using the current process's Pid is the key.

Further, while ec_date doesn't support PHP's timezone characters (e, I, O, P, T, Z, r, and c), qdate will handle them for us.

Exported Functions:

Conversion Functions

  • to_string(FormatString, ToTimezone, Date) - "FormatString" is a string that follows PHP's date function formatting rules. The date will be converted to the specified ToTimezone.
  • to_string(FormatString, Date) - same as to_string/3, but the Timezone is intelligently determined (see below)
  • to_string(FormatString) - same as to_string/2. but uses the current time as Date
  • to_date(ToTimezone, Date) - converts any date/time format to Erlang date format. Will first convert the date to the timezone ToTimezone.
  • to_date(Date) - same as to_date/2, but the timezone is determined (see below).
  • to_now(Date) - converts any date/time format to Erlang now format.
  • to_unixtime(Date) - converts any date/time format to a unixtime integer

A ToTimezone value of the atom auto will automatically determine the timezone. For example, to_date(Date, auto) is exactly the same as to_date(Date)

A Note About Argument Order: In all cases, ToTimezone is optional and if omitted, will be determined as described below in "Understanding Timezone Determining and Conversion". If ToTimezone is specified, it will always be immediately left of the Disambiguate argument (if it's specified), which is always immediately left of Date argument. Date will always be the last argument to any of the conversion and formatting functions.

Understanding Timezone Determining and Conversions

There is a lot of timezone inferring going on here.

If a Date string contains timezone information (i.e. "2008-12-21 6:00pm PST"), then qdate will parse that properly, determine the specified PST timezone, and do conversions based on that timezone. Further, you can specify a timezone manually, by specifying it as as a two-tuple for Date (see "Acceptable Date formats" above).

If no timezone is specified or determinable in a Date variable, then qdate will infer the timezone in the following order.

  • If specified by qdate:set_timezone(Timezone) for that process. Note, as specified below (in the "Timezone Functions" section), set_timezone/1 is a shortcut to set_timezone(self(), Timezone), meaning that set_timezone/1 only applies to that specific process. If none is specified.
  • If no timezone is specified for the process, qdate looks at the qdate application variable default_timezone.
  • If no timezone is specified by either of the above, qdate assumes "GMT" for all dates.
  • A timezone value of auto will act as if no timezone is specified.

Disambiguating Ambiguous Timezone Conversions

Sometimes, when youre converting a datetime from one timezone to another, there are potentially two different results if the conversion happens to land on in a timezone that's in the middle of a Daylight Saving conversion. For example, converting "11-Nov-2013 1:00:am" in "America/New York" to "GMT" could be both "5am" and "6am" in GMT, since "1am EST". This is a side effect of the "intelligence" of qdate - qdate would notice that 1am in New York is EST, and should be converted to "1am EST", and then do the conversion from "1am EST" to "GMT". This can lead to confusion.

Further, since qdate attempts to be "smart" about mistakenly entered timezones (ie, if you entered "2013-01-01 EDT", qdate knows that "EDT" (Eastern Daylight Time) doesn't apply to January first, so it assumes you meant "EST".

THE SOLUTION to this tangled mess that we call Daylight Saving Time is to provide an option to disambiguate if you so desire. By default disambiguation is disabled, and qdate will just guess as to it's best choice. But if you so desire, you can make sure qdate does both conversions, and returns both.

You can do this by passing a Disambiguation argument to to_string, to_date, to_unixtime, and to_now. Disambiguation can be an atom of the values:

  • prefer_standard (Default Behavior): If an ambiguous result occurs, qdate will return the date in standard time rather than daylight time.
  • prefer_daylight: If an ambiguous result occurs, qdate will return the preferred daylight time.
  • both: If an ambiguous result occurs, qdate will return the tuple: {ambiguous, DateStandard, DateDaylight}, where DateStandard is the date in Standard Time, and DateDaylight is the date in Daylight Saving Time.

So the expanded conversions functions are:

  • to_date(ToTimezone, Disambiguate, Date)
  • to_string(FormatString, ToTimezone, Disambiguate, Date)
  • to_unixtime(Disambiguate, Date)
  • to_now(Disambiguate, Date)

Examples:

1> qdate:set_timezone("GMT").
ok

%% Here, converting GMT 2013-11-03 6AM to America/New York yields an ambiguous
%% result
2> qdate:to_date("America/New York", both, {{2013,11,3},{6,0,0}}).
{ambiguous,{{2013,11,3},{1,0,0}},{{2013,11,3},{2,0,0}}}

%% Let's just use daylight time
3> qdate:to_date("America/New York", prefer_daylight, {{2013,11,3},{6,0,0}}).
{{2013,11,3},{2,0,0}}

%% Let's just use standard time (the default behavior)
4> qdate:to_date("America/New York", prefer_standard, {{2013,11,3},{6,0,0}}).
{{2013,11,3},{1,0,0}}

5> qdate:set_timezone("America/New York").
ok

%% Switching from 1AM Eastern Time to GMT yields a potentially ambiguous result
6> qdate:to_date("GMT", both, {{2013,11,3},{1,0,0}}).
{ambiguous,{{2013,11,3},{6,0,0}},{{2013,11,3},{5,0,0}}}

%% Use daylight time for conversion
7> qdate:to_date("GMT", prefer_daylight, {{2013,11,3},{1,0,0}}).
{{2013,11,3},{5,0,0}}

%% Here we demonstrated that even if we ask for "both", if there is no
%% ambiguity, the plain date is returned
8> qdate:to_date("GMT", both, {{2013,11,3},{5,0,0}}).
{{2013,11,3},{10,0,0}}

Conversion Functions provided for API compatibility with ec_date

  • parse/1 - Same as to_date(Date)
  • nparse/1 - Same as to_now(Date)
  • format/1 - Same as to_string/1
  • format/2 - Same as to_string/2

Date and Time Comparison

qdate provides a few convenience functions for performing date comparisons.

  • compare(A, B) - Like C's strcmp, returns:
    • 0: A and B are exactly the same.
    • -1: A is less than (before) B.
    • 1: A is greater than (after) B.
  • compare(A, Operator, B) - Operator is an infix comparison operator, and the function will return true if:
    • '=', or '==' - A is the same time as B
    • '/=', or '=/=' or '!=' - A is not the same time as B
    • '<' - A is before B
    • '>' - A is after B
    • '=<' or '<=' - A is before or equal to B
    • '>=' or '=>' - A is after or equal to B

Note 1: Operator must be an atom.

Note 2: These functions will properly compare times with different timezones (for example: compare("12am CST",'==',"1am EST") will properly return true)

Timezone Functions

  • set_timezone(Key, TZ) - Set the timezone to TZ for the key Key
  • set_timezone(TZ) - Sets the timezone, and uses the Pid from self() as the Key. Also links the process for removal from the record when the Pid dies.
  • get_timezone(Key) - Gets the timezone assigned to Key
  • get_timezone() - Gets the timezone using self() as the Key
  • clear_timezone(Key) - Removes the timezone record associated with Key.
  • clear_timezone() - Removes the timezone record using self() as Key. This function is not necessary for cleanup, most of the time, since if Key is a Pid, the qdate server will automatically clean up when the Pid dies.

Note: If no timezone is set, then anything relying on the timezone will default to GMT.

Registering Custom Parsers and Formatters

You can register custom parsers and formatters with the qdate server. This allows you to specify application-wide aliases for certain common formatting strings in your application, or to register custom parsing engines which will be attempted before engaging the ec_date parser.

Registering and Deregistering Parsers

  • register_parser(Key, ParseFun) - Registers a parsing function with the qdate server. ParseFun is expected to have the arity of 1, and is expected to return a DateTime format ({{Year,Month,Day},{Hour,Min,Sec}}) or, if your ParseFun is capable of parsing out a Timezone, the return the tuple {DateTime, Timezone}. Keep in mind, if your string already ends with a Timezone, the parser will almost certainly extract the timezone before it gets to your custom ParseFun. If your custom parser is not able to parse the string, then it should return undefined.
  • deregister_parser(Key) - If you previously registered a parser with the qdate server, you can deregister it by its Key.
  • get_parsers() - Get the list of all registered parsers and their keys.

Registering and Deregistering Formatters

  • register_format(Key, FormatString) - Register a formatting string with the qdate server, which can then be used in place of the typical formatting string.
  • deregister_format(Key) - Deregister the formatting string from the qdate server.
  • get_formats() - Get the list of all registered formats and their keys.

About backwards compatibility with ec_date and deterministic parsing

ec_date and dh_date both have a quirk that bothers me with respect to the parsing of dates that causes some date parsing to be non-deterministic. That is, if parsing an incomplete date or time (ie, a text string that is missing a time or a date), ec_date will automatically insert the current values of those as read by the system clock.

For example, if the following lines are run a few seconds apart:

1> ec_date:parse("2012-02-04").
{{2012,2,4},{0,1,10}}
2> ec_date:parse("2012-02-04").
{{2012,2,4},{0,1,12}}
3> ec_date:parse("2012-02-04").
{{2012,2,4},{0,1,13}}

As you can see, even though the inputs are the same each time, the resulting parsed dates have the current time inferred. The same behavior can be observed if parsing a time without a date:

4> ec_date:parse("7pm").
{{2013,4,30},{19,0,0}}

As you can see, even though the time did not specify a date, the resulting parsed datetime has the date inferred from the current date. Admittedly, inferring the date bothers me less than inferring the time, but in the name of consistency, there should be options for enabling or disabling both.

The Solution For Non-deterministic parsing

To solve this issue for users that are bothered by this, while preserving backwards compatibility for folks who prefer this, we're going to introduce a qdate application environment variable called deterministic_parsing.

The value of deterministic_parsing can be a tuple of the following format:

{DatePref, TimePref}

Where DatePref and TimePref are either of the following atoms:

  • now - Automatically fill in the missing date or time components with the current time (the is the behavior described above)
  • zero - Fill in the missing date or time components with zeroed out values. This means that if a date is missing, it'll be set to the unix epoch ({1970,1,1}) and if a time is missing, it'll be set to midnight: {0,0,0}.

So, the acceptable combinations can be the following:

  • {zero, zero} - Any missing components will be replaced with zero-values. (This is the qdate default behavior)
  • {now, zero} - If a date is missing, insert the current date, but if a time is missing, set it to midnight.
  • {zero, now} - If a date is missing, set it to the unix epoch, and if a time is missing, set it to the current time of day.
  • {now, now} - If either date or time are missing, set it to the current date or current time.

Note: If this application value is not set, the default behavior for qdate is to avoid non-determinism and use {zero, zero}.

To set this value, you can either set the value manually in code with:

application:set_env(qdate, deterministic_parsing, {now, zero}).

or (and this is the preferred method) use a config file and load it with

erl -config path/to/file.config

Sample config file specifying this application variable:

[{qdate, [
    {deterministic_parsing, {now, zero}}
]}].

Demonstration

Basic Conversion and Formatting

%% Let's start by making a standard Erlang DateTime tuple
1> Date = {{2013,12,21},{12,24,21}}.
{{2013,12,21},{12,24,21}}

%% Let's do a simple formatting of the date
2> DateString = qdate:to_string("Y-m-d h:ia", Date).
"2013-12-21 12:24pm"

%% We can also specify the format string as a binary
3> DateBinary = qdate:to_string(<<"Y-m-d h:ia">>,Date).
<<"2013-12-21 12:24pm">>

%% And we can parse the original string to get back a DateTime object
4> qdate:to_date(DateString).
{{2013,12,21},{12,24,0}}


%% We can do the same with a binary
5> qdate:to_date(DateBinary).
{{2013,12,21},{12,24,0}}

%% We can also parse that date and get a Unix timestamp
6> DateUnix = qdate:to_unixtime(DateString).
1387628640

%% And we can take that Unix timestamp and format it to a string
7> qdate:to_string("n/j/Y g:ia", DateUnix).
"12/21/2013 12:24pm"

%% We can take a date string and get an Erlang now() tuple
8> DateNow = qdate:to_now(DateString).
{1387,628640,0}

%% And we can convert it back

9> DateString2 = qdate:to_string("n/j/Y g:ia", DateNow).
"12/21/2013 12:24pm"

Note: That by this point, we've used, as the Date parameter, all natively supported date formats: Erlang datetime(), Erlang now(), Unix timestamp, and formatted text strings either as a list or as a binary.

For the most part, this will be the bread and butter usage of qdate. Easily converting from one format to another without having to worry about what format your data is currently in. qdate will figure it out for you.

But now, we're going to start getting interesting!

Registering Custom Parsers

%% Let's format our date into something shorter. This may, for example, be a
%% date format you may deal with when receiving a data-set from a client.
10> CompactDate = qdate:to_string("Ymd", DateNow).
"20131221"

%% Let's try to parse it
11> qdate:to_date(CompactDate).
** exception throw: {ec_date,{bad_date,"20131221"}}
     in function  ec_date:do_parse/3 (src/ec_date.erl, line 92)
     in call from qdate:to_date/2 (src/qdate.erl, line 169)

%% Well obviously, this isn't a standard format by any means, so it crashes.
%% You can parse it yourself before passing it to `qdate` or if you deal with
%% this format frequently enough, you can register it as a custom parser and
%% qdate will intelligently parse it if it can.

%% So let's make a simple parser for it that uses regular expressions:
12> ParseCompressedDate =
12>  fun(RawDate) when length(RawDate)==8 ->
12>       try re:run(RawDate,"^(\\d{4})(\\d{2})(\\d{2})$",[{capture,all_but_first,list}]) of
12>         nomatch -> undefined;
12>         {match, [Y,M,D]} ->
12>           ParsedDate = {list_to_integer(Y), list_to_integer(M), list_to_integer(D)},
12>           case calendar:valid_date(ParsedDate) of
12>              true -> {ParsedDate, {0,0,0}};
12>              false -> undefined
12>           end
12>       catch _:_ -> undefined
12>       end;
12>     (_) -> undefined
12>  end.
#Fun<erl_eval.6.82930912>

%% And now we'll register the parser with the `qdate` server, giving it a "Key"
%% of the atom 'compressed_date'
13> qdate:register_parser(compressed_date,ParseCompressedDate).
compressed_date

%% Now, let's try parsing that again
14> qdate:to_date(CompactDate).
{{2013,12,21},{0,0,0}}

%% Huzzah! It worked. From here on out, `qdate`, will properly parse that kind
%% of data if that format is passed, otherwise, it will merely skip over that
%% parser and engage the standard parser in `ec_date`

Note: Currently, qdate expects custom parsers to not crash. If a custom parser crashes, an exception will be thrown. This is done in order to help you debug your parsers. If a parser receives an unexpected input and crashes, the exception will be generated and you will be able to track down what input caused the crash.

Another Note: Custom parsers are expected to return either:

  • A datetime() tuple. (ie {{2012,12,21},{14,45,23}}).
  • The atom undefined if this parser is not a match for the supplied value

Registering Custom Formats

%% Let's format a date to a rather long string
15> qdate:to_string("l, F jS, Y g:i A T",DateString).
"Saturday, December 12st, 2013 12:24 PM GMT"

%% Boy, that sure was a long string, I hope you can remember all those
%% characters in that order!

%% But, you don't have to: if that's a common format you use in your
%% application, you can register your format with the `qdate` server, and then
%% easiy refer to that format by its key.

%% So let's take that format and register it
16> qdate:register_format(longdate, "l, F jS, Y g:i A T").
ok

%% Now, let's try to format our string 
17> LongDateString = qdate:to_string(longdate, DateString).
"Saturday, December 21st, 2013 12:24 PM GMT"

%% It was certainly easier to remember the atom 'longdate' than trying to
%% remember the seemingly random "l, F jS, Y g:i A T".

Ain't it nice, making things easier for you?

Timezone Demonstrations

The observant reader would have noticed something else. We used timezones in the last couple of calls. Indeed, not only can qdate deal with formatting timezones, but it can also parse them, convert them, and set them for simplified conversions.

Let's see how we do this

%% Let's take that last long date string (that was in GMT) and move it to
%% Pacific time
18> LongDatePDT = qdate:to_string(longdate, "PDT", LongDateString).
"Saturday, December 21st, 2013 4:24 AM PST"

%% See something interesting there? Yeah, we told it it was PDT, but it output
%% PST.  That's because PST is not in daylight saving time in December, and 
%% `qdate` was able to intelligently infer that, and fix it for us.

%% Note, that when in doubt, `qdate` will *not* convert. For example, not all
%% places in Eastern Standard Time do daylight saving time, and as such, EST
%% will not necessarily convert to EDT.

%% However, if you provide the timezone as something like "America/New York",
%% it *will* figure that out, and do the correct conversion for you. 

%% Let's see how it handles unix times with strings that contain timezones.
%% If you recall, LongDateString = "Saturday, December 21st, 2013 12:24 PM GMT"
%% and LongDatePDT = "Saturday, December 21st, 2013 4:24 AM PST"
19> qdate:to_unixtime(LongDateString).
1387628640

%% Now let's try it with the Pacific Time one
20> qdate:to_unixtime(LongDatePDT).
1387628640

%% How exciting! `qdate` properly returned the same unix timestamp, since unix
%% timestamps are timezone neutral. That is because unix timestamps are the
%% number of seconds since midnight on 1970-01-01 GMT. As such, unix timestamps
%% should not change, just because you're in a different timezone.

%% Let's set the timezone for the current process to EST to test that previous
%% assertion
21> qdate:set_timezone("EST").
ok

%% Now let's try converting those dates to unixtimes again
22> qdate:to_unixtime(LongDateString).
1387628640
23> qdate:to_unixtime(LongDatePDT).
1387628640

%% Great! They didn't change, as we expected. The unix timestamps have remained
%% Timezone neutral.

%% Let's clear the current process's timezone (which basically means setting it
%% to the application variable `default_timezone`, or, in this case, just
%% resetting it to "GMT"
24> qdate:clear_timezone().
ok

%% Now, let's imagine you run a website. The main site may have its own
%% timezone, and the users each also have their own timezones.  So we'll
%% register timezones for each the main site, and each user. That way, if we
%% need to ensure that a date is presented in an appropriate timezone.


%% Let's register some timezones by "Timezone Keys".  
25> qdate:set_timezone(my_site, "America/Chicago").
ok
26> qdate:set_timezone({user,1},"Australia/Melbourne").
ok

%% So we'll get the date object of the previously set unix timestamp `DateUnix`
27> qdate:to_date(DateUnix).
{{2013,12,21},{12,24,0}}

%% And let's format it, also showing the timezone offset that was used
28> qdate:to_string("Y-m-d H:i P", DateUnix).
"2013-12-21 12:24 +00:00"

%% Since we cleared the timezone for the current process, it just used "GMT"

%% Let's get the date again, but this time, use to the Timezone key `my_site`
29> qdate:to_date(my_site, DateUnix).
{{2013,12,21},{6,24,0}}

%% And let's format it to show again the timezone offset
30> qdate:to_string("Y-m-d H:i P", my_site, DateUnix).
"2013-12-21 06:24 -06:00"

%% Finally, let's get the date using the User's timezone key
31> qdate:to_date({user,1}, DateUnix).
{{2013,12,21},{23,24,0}}

%% And again, formatted to show the timezone offset
32> UserDateWithHourOffset = qdate:to_string("Y-m-d H:i P", {user,1}, DateUnix).
"2013-12-21 23:24 +11:00"

%% And finally, let's just test some more parsing and converting. Here, despite
%% the fact that the timezone is presented as "+11:00", `qdate` is able to
%% do the proper conversion, and give us back the same unix timestamp that was
%% used.
33> qdate:to_unixtime(UserDateWithHourOffset).
1387628640

One last bit of magic that may confuse you without an explanation

Magic is usually bad, you know what's worse? Timezones and Daylight Saving Time. So we use a little magic to try and simplify them for us. Below is the extent of the confusion with related to inferring timezones and formatting dates

%% First, let's set the timezone to something arbitrary
34> qdate:set_timezone("EST").
ok

%% Let's convert this date to basically the same time format, just without the
%% timezone identifier.
35> qdate:to_string("Y-m-d H:i","2012-12-21 00:00 PST").
"2012-12-21 03:00"

%% WHAT?! We entered a date and time, and out came a different time?!
%% I CALL SHENANIGANS!

%% Let's add that timezone indicator back in with the conversion to see what
%% happened:

36> qdate:to_string("Y-m-d H:i T","2012-12-21 00:00 PST").
"2012-12-21 03:00 EST"

%% OOOOOOOHHH! I see!
%% Because we set our current timezone to EST, it took the original provided
%% date in PST, and converted it to EST (since EST is the timezone we've chosen
%% for the current process). So it's taking whatever date, and if it can
%% determine a timezone, it'll extract that timezone, and convert the time from
%% that timezone to our intended timezone.

Date Arithmetic

(not fully tested yet, but will have full tests for 0.4.0)

The current implementation of qdate's date arithmetic returns Unixtimes.

There are 8 main functions for date arithmetic:

+ `add_seconds(Seconds, Date)`
+ `add_minutes(Minutes, Date)`
+ `add_hours(Hours, Date)`
+ `add_days(Days, Date)`
+ `add_weeks(Weeks, Date)`
+ `add_months(Months, Date)`
+ `add_years(Years, Date)`
+ `add_date(DateToAdd, Date)` - `DateToAdd` is a shortcut way of adding
  numerous options. For example. `qdate:add_date({{1, 2, -3}, {-500, 20, 0}})`
  will add 1 year, add 2 months, subtract 3 days, subtract 500 hours, add 20
  minutes, and not make any changes to seconds.

For the date arithmetic functions, Date, like all qdate functions, can be any format.

Date Arithmetic from "now"

There are 7 other arithmetic functions that take a single argument, and these do arithmetic from "now." For example, add_years(4) is a shortcut for add_years(4, os:timestamp()).

  • add_seconds(Seconds)
  • add_minutes(Minutes)
  • add_hours(Hours)
  • add_days(Days)
  • add_weeks(Weeks)
  • add_months(Months)
  • add_years(Years)

Thanks

A few shoutouts to Dale Harvey and the Erlware team for dh_date/ec_date, and to Dmitry Melnikov for the erlang_localtime package. Without the hard work of all involved in those projects, qdate would not exist.

Thanks to Additional Contributors

Changelog

See CHANGELOG.markdown

TODO

  • Make qdate backend-agnostic (allow specifying either ec_date or dh_date as the backend)
  • Add -spec and -type info for dialyzer
  • Provide a sample qdate.config for users to see
  • Research the viability of ezic for a timezone backend replacement for erlang_localtime.

Conclusion

I hope you find qdate helpful in all your endeavors and it helps make your wildest dreams come true!

If you have any bugs, feature requests, or whatnot, feel free to post a Github issue, ping me on Twitter, or email me below.

I'm open to pull requests. Feel free to get your hands dirty!

Author: Jesse Gumm

Email: gumm@sigma-star.com

Twitter: @jessegumm

Released under the MIT License (see LICENSE file)

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