local-time.texinfo

\input texinfo   @c -*- Mode: Texinfo; Mode: auto-fill -*-
@c %**start of header
@setfilename local-time.info
@settitle The local-time Manual
@exampleindent 2

@c @documentencoding utf-8

@macro name {}
@code{local-time}
@end macro

@macro mathx {tex, non-tex}
@iftex
@math{\tex\}
@end iftex
@ifnottex
@emph{\non-tex\}
@end ifnottex
@end macro

@macro impnote {text}
@quotation Implementor's note
@emph{\text\}
@end quotation
@end macro

@c Set ROMANCOMMENTS to get comments in roman font.
@ifset ROMANCOMMENTS
@alias lispcmt = r
@end ifset
@ifclear ROMANCOMMENTS
@alias lispcmt = asis
@end ifclear

@c Index for public api.
@defindex it

@macro pub {name}
@itindex \name\
@c
@end macro

@macro k {what}
@code{\what\}
@end macro

@iftex
@alias v = asis
@alias cl = code
@end iftex

@ifnottex
@alias v = var
@alias cl = strong
@end ifnottex

@c Show variables, clauses, and concepts in the same index.
@syncodeindex it cp
@syncodeindex vr cp

@copying
Copyright @copyright{} 2012 Daniel Lowe <dlowe dlowe.net> @*
Copyright @copyright{} 2012 Attila Lendvai <attila.lendvai gmail.com> @*

@quotation
This manual describes the @name{} Common Lisp library which is
based on Erik Naggum's  @emph{The Long, Painful History of Time}
[NaggumPaper] paper.

@end quotation
@end copying
@c %**end of header

@titlepage
@title The @name{} Manual
@subtitle Version 1.0.0
@author Daniel Lowe
@author Attila Lendvai

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top @name{}
@insertcopying
@end ifnottex

@menu
* Introduction::
* Public API::
* Other Features::
* References::
* Comprehensive Index::
@end menu

@c ===================================================================
@node Introduction
@chapter Introduction

The @name{} library is a Common Lisp library for the manipulation of
dates, times and intervals. It was originally based almost entirely
upon Erik Naggum's paper @emph{The Long Painful History of Time}
[NaggumPaper].  Many of the core concepts originated from this paper,
such as the seperation of days and seconds, the choice of 2000-03-01
as the standard epoch, and the timestring format.

@quotation Caveats
This implementation assumes that time zone information is stored in
the tzfile format. The default timezone is loaded from
/etc/localtime. On non-POSIX systems, this will certainly give
different results than the system time handling.
@end quotation

@c ===================================================================
@node Public API
@chapter Public API

@menu
* Types::
* Timezones::
* Creating timestamp Objects::
* Querying timestamp Objects::
* Manipulating Date and Time Values::
* Parsing and Formatting::
@end menu

@c ===================================================================
@node Types
@section Types

It's a good idea to treat all values as immutable objects. @name{}
will not modify any object it was given unless explicitly asked to by
the @code{:into} keyword argument.

@itindex timestamp
@deftp Class timestamp day sec nsec

@code{timestamp} values can represent either a @emph{date},
a @emph{daytime} or a @emph{time} value. It has the following slots:

@lisp
(defclass timestamp ()
  ((day :type integer)
   (sec :type integer)
   (nsec :type (integer 0 999999999))))
@end lisp

The following constraints apply to the specific types:
@itemize

@item
@emph{date}: must have a @var{+utc-zone+} timezone and the @var{sec}
slot must be the first second of a day; In other words, the time
elements of the @code{timestamp} value must have their least possible
values.

@item
@emph{time}: the @var{day} slot must be zero.

@end itemize
@end deftp

@deftp Struct timezone path name loaded

@code{timezone} objects represent timezones - local and political
modifications to the time representation.  Timezones are responsible
for storing offsets from GMT, abbreviations for different
sub-timezones, and the times each sub-timezone is to be in effect.

@end deftp

@c ===================================================================
@node Timezones
@section Timezones

@itindex *default-timezone*
@defvr Default *default-timezone*

The variable @var{*default-timezone*} contains the timezone that will
be used by default if none is specified.  It is loaded from
@emph{/etc/localtime} when the library is loaded.  If
@emph{/etc/localtime} is not present, it will default to UTC.

@end defvr

@itindex +utc-time+
@defvr Constant +utc-time+

The variable @var{+utc-zone+} contains a timezone corresponding to
UTC.

@end defvr


@itindex define-timezone
@defmac define-timezone zone-name zone-file &key (load nil)

Define @var{zone-name} (a symbol or a string) as a new timezone,
lazy-loaded from @var{zone-file} (a pathname designator relative to
the zoneinfo directory on this system.  If @var{load} is true, load
immediately.

@end defmac

@c ===================================================================
@node Creating timestamp Objects
@section Creating @code{timestamp} Objects

@itindex universal-to-timestamp
@defun universal-to-timestamp universal &key (nsec 0)

Produces a @code{timestamp} instance from the provided universal time
@var{universal}.  Universal time is defined in the Common Lisp
Specification as the number of seconds since 1900-01-01T00:00:00Z.
@end defun


@itindex unix-to-timestamp
@defun unix-to-timestamp unix &key (nsec 0)

Produces a @code{timestamp} instance from the provided unix time
@var{unix}.  Unix time is defined by POSIX as the number of seconds
since 1970-01-01T00:00:00Z.
@end defun


@itindex now
@defun now

Produces a @code{timestamp} instance with the current time.  Under
sbcl, the new timestamp will be precise to the microsecond.
Otherwise, the precision is limited to the second.
@end defun


@itindex today
@defun today

Produces a @code{timestamp} instance that corresponds to today's
date, which is the midnight of the current day in the UTC zone.
@end defun


@itindex encode-timestamp
@defun encode-timestamp nsec sec minute hour day month year &key timezone offset into

Returns a new @code{timestamp} instance corresponding to the specified
time elements.  The @var{offset} is the number of seconds offset from
UTC of the locale.  If @var{offset} is not specified, the offset will
be guessed from the @var{timezone}.  If a @code{timestamp} is passed
as the @var{into} argument, its value will be set and that
@code{timestamp} will be returned.  Otherwise, a new @code{timestamp}
is created.
@end defun


@itindex make-timestamp
@defmac make-timestamp &key :day :sec :nsec

Expands to an expression that creates an instance of a
@code{timestamp} exactly as specified.
@end defmac


@itindex clone-timestamp
@defmac clone-timestamp timestamp

Expands to an expression that creates another copy of @var{timestamp}
that is @code{timestamp=} to it.
@end defmac


@c ===================================================================
@node Querying timestamp Objects
@section Querying @code{timestamp} Objects

@itindex day-of
@defun day-of timestamp

Returns the day component of @var{timestamp}.  Although Naggum's paper
specifies that the day should be a signed fixnum, it is left unbounded
for flexibility reasons.
@end defun

@itindex sec-of
@defun sec-of timestamp

Returns the 'seconds' component of the time.  Valid values for the
seconds range from 0 to 86399.
@end defun

@itindex nsec-of
@defun nsec-of timestamp

Returns the 'microseconds' component of the time.  Valid values for
the nanoseconds range from 0 to 999999999.
@end defun

@itindex timestamp-to-universal
@defun timestamp-to-universal timestamp

This returns the date/time specified in @var{timestamp} encoded as
the number of seconds since January 1st, 1900 12:00am UTC.
@end defun

@itindex timestamp-to-unix
@defun timestamp-to-unix timestamp

This returns the date/time specified in @var{timestamp} encoded as
the number of seconds since January 1st, 1970 12:00am UTC.  It
corresponds with the time received from the POSIX call @code{time()}.
@end defun

@itindex timestamp-subtimezone
@defun timestamp-subtimezone timestamp timezone

Returns as multiple values the time zone applicable at the given time
as the number of seconds east of UTC, a boolean daylight-saving-p, and
the customary abbreviation of the timezone.
@end defun


@itindex with-decoded-timestamp
@defmac with-decoded-timestamp (&key nsec sec minute hour day month year day-of-week daylight-p timezone) timestamp &body body

This macro binds variables to the decoded elements of @var{timestamp}.
The timezone argument is used for decoding the timestamp, and is not
bound by the macro. The value of @var{day-of-week} starts from 0 which
means Sunday.
@end defmac


@itindex decode-timestamp
@defun decode-timestamp timestamp

Returns the decoded time as @code{(values ns ss mm hh day month
year day-of-week daylight-saving-time-p timezone-offset timezone-abbreviation)}.
@end defun


@itindex timestamp<
@itindex timestamp<=
@itindex timestamp>
@itindex timestamp>=
@itindex timestamp=
@itindex timestamp/=
@defun timestamp< time-a time-b
@defunx timestamp<= time-a time-b
@defunx timestamp> time-a time-b
@defunx timestamp>= time-a time-b
@defunx timestamp= time-a time-b
@defunx timestamp/= time-a time-b

These comparison functions act like their string and char counterparts.
@end defun


@itindex timestamp-minimum
@defun timestamp-minimum timestamp &rest timestamps

Returns the earliest timestamp passed to it.
@end defun


@itindex timestamp-maximum
@defun timestamp-maximum timestamp &rest timestamps

Returns the latest timestamp passed to it.
@end defun


@itindex timestamp-day-of-week
@defun timestamp-day-of-week timestamp

This returns the index of the day of the week,
starting at 0 which means Sunday.
@quotation Note
''Day of the week'' is ambigous and locale dependent.
@end quotation
@end defun


@itindex universal-to-timestamp
@defun universal-to-timestamp timestamp

Returns the UNIVERSAL-TIME corresponding to @var{timestamp}.
@quotation Note
Subsecond precision is not preserved.
@end quotation
@end defun


@itindex timestamp-millennium
@itindex timestamp-century
@itindex timestamp-decade
@defun timestamp-millennium timestamp &key timezone
@defunx timestamp-century timestamp &key timezone
@defunx timestamp-decade timestamp &key timezone

Returns the ordinal millennium, century or decade upon which the
timestamp falls.  Ordinal time values start at 1, so the
(timestamp-century (now)) will return 21.

@end defun

@itindex timestamp-year
@itindex timestamp-month
@itindex timestamp-day
@itindex timestamp-hour
@itindex timestamp-minute
@itindex timestamp-second
@itindex timestamp-millisecond
@itindex timestamp-microsecond
@defun timestamp-year timestamp &key timezone
@defunx timestamp-month timestamp &key timezone
@defunx timestamp-day timestamp &key timezone
@defunx timestamp-hour timestamp &key timezone
@defunx timestamp-minute timestamp &key timezone
@defunx timestamp-second timestamp &key timezone
@defunx timestamp-millisecond timestamp &key timezone
@defunx timestamp-microsecond timestamp &key timezone
@defunx timestamp-microsecond timestamp &key timezone

Returns the decoded part of the timestamp.
@end defun


@c ===================================================================
@node Manipulating Date and Time Values
@section Manipulating Date and Time Values

@itindex timestamp+
@itindex timestamp-
@defun timestamp+ time amount unit
@defunx timestamp- time amount unit

Add or subtract the @var{amount} to the @var{time} using the specified
@var{unit}.  @var{unit} may be one of ( @code{:nsec} @code{:sec}
@code{:minute} @code{:hour} @code{:day} @code{:month} @code{:year}).
The value of the parts of the timestamp of higher resolution than the
UNIT will never be touched.  If you want a precise number of seconds
from a time, you should specify the offset in seconds.
@end defun

@itindex timestamp-maximize-part
@defun timestamp-maximize-part timestamp part &key offset timezone into

Returns a timestamp with its parts maximized up to @var{part}.  @var{part} can be
any of (:nsec :sec :min :hour :day :month).  If @var{into} is specified, it
will be modified and returned, otherwise a new timestamp will be
created.
@end defun


@itindex timestamp-minimize-part
@defun timestamp-minimize-part timestamp part &key offset timezone into

Returns a timestamp with its parts minimized up to @var{part}.  @var{part} can be
any of (:nsec :sec :min :hour :day :month).  If @var{into} is specified, it
will be modified and returned, otherwise a new timestamp will be
created.
@end defun


@itindex adjust-timestamp
@defmac adjust-timestamp timestamp &body changes

Alters various parts of @var{timestamp}, given a list of changes.  The
changes are in the format @code{(offset part value)} and @code{(set
part value)}.

@lisp
;; Return a new @code{timestamp} value that points to the previous Monday
(adjust-timestamp (today) (offset :day-of-week :monday))

;; Return a new @code{timestamp} value that points three days ahead from now
(adjust-timestamp (today) (offset :day 3))
@end lisp

Keep in mind that @code{adjust-timestamp} is not a mere setter for
fields but instead it handles overflows and timezone conversions as
expected. Also note that it's possible to specify multiple commands.

The list of possible places to manipulate are: @code{:nsec}
@code{:sec} @code{:sec-of-day} @code{:minute} @code{:hour}
@code{:day} @code{:day-of-month} @code{:month} @code{:year}.
@end defmac


@itindex adjust-timestamp!
@defmac adjust-timestamp! timestamp &body changes

Just like @code{adjust-timestamp}, but instead of returning a freshly
constructed value, it alters the provided @var{timestamp} value (and
returns it).
@end defmac


@itindex timestamp-whole-year-difference
@defun timestamp-whole-year-difference time-a time-b

Returns the number of whole years elapsed between @var{time-a} and @var{time-b}.
@quotation Note
This is useful for calculating anniversaries and birthdays.
@end quotation
@end defun


@itindex days-in-month
@defun days-in-month month year

Returns the number of days in a given month of the specified year.
@end defun


@c ===================================================================
@node Parsing and Formatting
@section Parsing and Formatting


@itindex +iso-8601-format+
@defvr Constant +iso-8601-format+

The constant @var{+iso-8601-format+} is bound to a description of the ISO 8601 format.  An output with this format will look like this: @samp{2008-03-01T19:42:34.608506+01:00}.  This is the default format for the @code{format-timestring} function.

@end defvr

@itindex +asctime-format+
@defvr Constant +asctime-format+

The constant @var{+asctime-format+} is bound to a format mirroring the output of the POSIX asctime() function.  An output with this format will look like this: @samp{Sat Mar  1 19:42:34 2008}.

@end defvr

@itindex +rfc-1123-format+
@defvr Constant +rfc-1123-format+

The constant @var{+rfc-1123-format+} is bound to a description of the format defined in RFC 1123 for Internet timestamps.  An output with this format will look like this: @samp{Sat, 01 Mar 2008 19:42:34 EDT}.

@end defvr

@itindex +iso-week-date-format+
@defvr Constant +iso-week-date-format+

The constant @var{+iso-week-date-format+} is bound to a description of the ISO 8601 Week Date format.  An output with this format will look like this: @samp{2009-W53-5}.

@end defvr

@itindex parse-timestring
@defun parse-timestring timestring &key (start 0) end (fail-on-error t) (offset 0)

Parses a timestring and returns the corresponding @code{timestamp}.
Parsing begins at @var{start} and stops at the @var{end}
position. If there are invalid characters within @code{timestring}
and @var{fail-on-error} is @code{T}, then an @code{invalid-timestring}
error is signaled, otherwise @code{NIL} is returned.

If there is no timezone specified in @code{timestring} then
@var{offset} is used as the default timezone offset (in seconds).
@end defun


@itindex format-timestring
@defun format-timestring (destination timestamp &key (format +iso-8601-format+) (timezone *default-timezone*))

Constructs a string representation of TIMESTAMP according to FORMAT and returns it.  If destination is T, the string is written to *standard-output*.  If destination is a stream, the string is written to the stream.

FORMAT is a list containing one or more of strings, characters, and keywords.  Strings and characters are output literally, while keywords are replaced by the values here:

@table @code
@item :year
*year
@item :month
*numeric month
@item :day
*day of month
@item :weekday
*numeric day of week, starting from 0 which means Sunday
@item :hour
*hour
@item :min
*minutes
@item :sec
*seconds
@item :msec
*milliseconds
@item :usec
*microseconds
@item :nsec
*nanoseconds
@item :iso-week-year
*year for ISO week date (can be different from regular calendar year)
@item :iso-week-number
*ISO week number (i.e. 1 through 53)
@item :iso-week-day
*ISO compatible weekday number (i.e. monday=1, sunday=7)
@item :ordinal-day
day of month as an ordinal (e.g. 1st, 23rd)
@item :long-weekday
long form of weekday (e.g. Sunday, Monday)
@item :short-weekday
short form of weekday (e.g. Sun, Mon)
@item :long-month
long form of month (e.g. January, February)
@item :short-month
short form of month (e.g. Jan, Feb)
@item :hour12
hour on a 12-hour clock
@item :ampm
am/pm marker in lowercase
@item :gmt-offset
the gmt-offset of the time, in +00:00 form
@item :gmt-offset-or-z
like :gmt-offset, but is Z when UTC
@item :timezone
timezone abbrevation for the time
@end table

Elements marked by * can be placed in a list in the form:
@lisp
(:keyword padding &optional (padchar #\0))
@end lisp
The string representation of the value will be padded with the padchar.

You can see examples by examining the values in @var{+iso-8601-format+}, @var{+asctime-format+}, and @var{+rfc-1123-format+}.

Produces on @var{stream} the timestring corresponding to the @var{timestamp} with
the given options.  If @var{stream} is @code{nil}, only returns a string containing what
would have been the output.  If @var{stream} is @code{t}, prints the string to
@var{*standard-output*}.

Example output:
@lisp
LOCAL-TIME> (format-timestring nil (now))
"2008-03-01T19:42:34.608506+01:00"
@end lisp
@end defun


@itindex format-rfc3339-timestring
@defun format-rfc3339-timestring (destination timestamp &key omit-date-part omit-time-part omit-timezone-part (use-zulu t))

Formats the time like format-timestring, but in RFC 3339 format. The options control valid options in the RFC.
@end defun



@c ===================================================================
@node Other Features
@chapter Other Features

@section Reader Macros

@itindex enable-read-macros
@defun enable-read-macros

Adds @@TIMESTRING and #@@UNIVERSAL-TIME as reader macros.
@end defun


@section Support for non-Gregorian Calendars

@itindex astronomical-julian-date
@defun astronomical-julian-date timestamp

Returns the julian date of the date portion of @var{timestamp}.
@end defun


@itindex modified-julian-date
@defun astronomical-julian-date timestamp

Returns the modified julian date of the date portion of @var{timestamp}.
@end defun

@c ===================================================================
@node References
@chapter References

@itemize

@item
[NaggumPaper] Erik Naggum. @emph{The Long Painful History of Time}
@url{http://naggum.no/lugm-time.html}, 1999.

@end itemize


@c ===================================================================
@node Comprehensive Index
@unnumbered Index
@printindex cp

@bye