local-time
**********

Copyright © 2008 Daniel Lowe <dlowe bitmuse.com>

Copyright © 2008 Attila Lendvai <attila.lendvai gmail.com>

     This manual describes the local-time Common Lisp library which is based on Erik Naggum's  The Long, Painful History of Time [NaggumPaper] paper.

Introduction
************

The local-time 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 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.

     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.

Public API
**********

Types
=====

It's a good idea to treat all values as immutable objects. local-time will not modify any object it was given unless explicitly asked to by the :into keyword argument.

 -- Class: timestamp day sec nsec

     timestamp values can represent either a date, a daytime or a time value. It has the following slots:

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

     The following constraints apply to the specific types:

       * date: must have a +UTC-ZONE+ timezone and the SEC slot must be the first second of a day; In other words, the time elements of the timestamp value must have their least possible values.

       * time: the DAY slot must be zero.

 -- Struct: timezone path name loaded

     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.

Timezones
=========

 -- Default: *default-timezone*

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

 -- Constant: +utc-time+

     The variable +UTC-ZONE+ contains a timezone corresponding to UTC.

 -- Macro: define-timezone zone-name zone-file &key (load nil)

     Define ZONE-NAME (a symbol or a string) as a new timezone, lazy-loaded from ZONE-FILE (a pathname designator relative to the zoneinfo directory on this system.  If LOAD is true, load immediately.

Creating timestamp Objects
==========================

 -- Function: universal-to-timestamp universal &key (nsec 0)

     Produces a timestamp instance from the provided universal time UNIVERSAL.  Universal time is defined in the Common Lisp Specification as the number of seconds since 1900-01-01T00:00:00Z.

 -- Function: unix-to-timestamp unix &key (nsec 0)

     Produces a timestamp instance from the provided unix time UNIX.  Unix time is defined by POSIX as the number of seconds since 1970-01-01T00:00:00Z.

 -- Function: now

     Produces a 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.

 -- Function: today

     Produces a timestamp instance that corresponds to today's date, which is the midnight of the current day in the UTC zone.

 -- Function: encode-timestamp nsec sec minute hour day month year &optional offset

     Returns a new timestamp instance corresponding to the specified time elements.  The OFFSET is the number of seconds offset from UTC of the locale.  The offset will be set by default to the lisp implementation's default offset at the current time.

 -- Macro: make-timestamp &key :day :sec :nsec

     Expands to an expression that creates an instance of a timestamp exactly as specified.

 -- Macro: clone-timestamp timestamp

     Expands to an expression that creates another copy of TIMESTAMP that is timestamp= to it.

Querying timestamp Objects
==========================

 -- Function: day-of timestamp

     Returns the day component of TIMESTAMP.  Although Naggum's paper specifies that the day should be a signed fixnum, it is left unbounded for flexibility reasons.

 -- Function: sec-of timestamp

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

 -- Function: nsec-of timestamp

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

 -- Function: timestamp-to-universal timestamp

     This returns the date/time specified in TIMESTAMP encoded as the number of seconds since January 1st, 1900 12:00am UTC.

 -- Function: timestamp-to-unix timestamp

     This returns the date/time specified in TIMESTAMP encoded as the number of seconds since January 1st, 1970 12:00am UTC.  It corresponds with the time received from the POSIX call time().

 -- Function: timestamp-subtimezone timestamp &optional 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.

 -- Macro: 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 TIMESTAMP. The timezone argument is used for decoding the timestamp, and is not bound by the macro. The value of DAY-OF-WEEK starts from 0 which means Sunday.

 -- Function: decode-timestamp timestamp

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

 -- Function: timestamp< time-a time-b
 -- Function: timestamp<= time-a time-b
 -- Function: timestamp> time-a time-b
 -- Function: timestamp>= time-a time-b
 -- Function: timestamp= time-a time-b
 -- Function: timestamp/= time-a time-b

     These comparison functions act like their string and char counterparts.

 -- Function: timestamp-minimum timestamp &rest timestamps

     Returns the earliest timestamp passed to it.

 -- Function: timestamp-maximum timestamp &rest timestamps

     Returns the latest timestamp passed to it.

 -- Function: timestamp-day-of-week timestamp

     This returns the index of the day of the week, starting at 0 which means Sunday.

          ''Day of the week'' is ambigous and locale dependent.

 -- Function: universal-to-timestamp timestamp

     Returns the UNIVERSAL-TIME corresponding to TIMESTAMP.

          Subsecond precision is not preserved.

 -- Function: timestamp-millennium timestamp &key timezone
 -- Function: timestamp-century timestamp &key timezone
 -- Function: 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.

 -- Function: timestamp-year timestamp &key timezone
 -- Function: timestamp-month timestamp &key timezone
 -- Function: timestamp-day timestamp &key timezone
 -- Function: timestamp-hour timestamp &key timezone
 -- Function: timestamp-minute timestamp &key timezone
 -- Function: timestamp-second timestamp &key timezone
 -- Function: timestamp-millisecond timestamp &key timezone
 -- Function: timestamp-microsecond timestamp &key timezone
 -- Function: timestamp-microsecond timestamp &key timezone

     Returns the decoded part of the timestamp.

Manipulating Date and Time Values
=================================

 -- Function: timestamp+ time amount unit
 -- Function: timestamp- time amount unit

     Add or subtract the AMOUNT to the TIME using the specified UNIT.  UNIT may be one of ( :nsec :sec :minute :hour :day :month :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.

 -- Function: timestamp-maximize-part timestamp part &key offset timezone into

     Returns a timestamp with its parts maximized up to PART.  PART can be any of (:nsec :sec :min :hour :day :month).  If INTO is specified, it will be modified and returned, otherwise a new timestamp will be created.

 -- Function: timestamp-minimize-part timestamp part &key offset timezone into

     Returns a timestamp with its parts minimized up to PART.  PART can be any of (:nsec :sec :min :hour :day :month).  If INTO is specified, it will be modified and returned, otherwise a new timestamp will be created.

 -- Macro: adjust-timestamp timestamp &body changes

     Alters various parts of TIMESTAMP, given a list of changes.  The changes are in the format (offset part value) and (set part value).

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

          ;; Return a new timestamp value that points three days ahead from now
          (adjust-timestamp (today) (offset :day 3))

     Keep in mind that 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: :nsec :sec :sec-of-day :minute :hour :day :day-of-month :month :year.

 -- Macro: adjust-timestamp! timestamp &body changes

     Just like adjust-timestamp, but instead of returning a freshly constructed value, it alters the provided TIMESTAMP value (and returns it).

 -- Function: timestamp-whole-year-difference time-a time-b

     Returns the number of whole years elapsed between TIME-A and TIME-B.

          This is useful for calculating anniversaries and birthdays.

 -- Function: days-in-month month year

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

Parsing and Formatting
======================

 -- Constant: +iso-8601-format+

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

 -- Constant: +asctime-format+

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

 -- Constant: +rfc-1123-format+

     The constant +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: Sat, 01 Mar 2008 19:42:34 EDT.

 -- Function: parse-timestring timestring &key (start 0) end (fail-on-error t) (offset 0)

     Parses a timestring and returns the corresponding timestamp. Parsing begins at START and stops at the END position. If there are invalid characters within timestring and FAIL-ON-ERROR is T, then an invalid-timestring error is signaled, otherwise NIL is returned.

     If there is no timezone specified in timestring then OFFSET is used as the default timezone offset (in seconds).

 -- Function: 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:

     :year
          *year

     :month
          *numeric month

     :day
          *day of month

     :weekday
          *numeric day of week, starting from 0 which means Sunday

     :hour
          *hour

     :min
          *minutes

     :sec
          *seconds

     :msec
          *milliseconds

     :usec
          *microseconds

     :nsec
          *nanoseconds

     :ordinal-day
          day of month as an ordinal (e.g. 1st, 23rd)

     :long-weekday
          long form of weekday (e.g. Sunday, Monday)

     :short-weekday
          short form of weekday (e.g. Sun, Mon)

     :long-month
          long form of month (e.g. January, February)

     :short-month
          short form of month (e.g. Jan, Feb)

     :hour12
          hour on a 12-hour clock

     :ampm
          am/pm marker in lowercase

     :gmt-offset
          the gmt-offset of the time, in +00:00 form

     :gmt-offset-or-z
          like :gmt-offset, but is Z when UTC

     :timezone
          timezone abbrevation for the time

     Elements marked by * can be placed in a list in the form:

          (:keyword padding &optional (padchar #\0))

     The string representation of the value will be padded with the padchar.

     You can see examples by examining the values in +ISO-8601-FORMAT+, +ASCTIME-FORMAT+, and +RFC-1123-FORMAT+.

     Produces on STREAM the timestring corresponding to the TIMESTAMP with the given options.  If STREAM is nil, only returns a string containing what would have been the output.  If STREAM is t, prints the string to *STANDARD-OUTPUT*.

     Example output:

          LOCAL-TIME> (format-timestring nil (now))
          "2008-03-01T19:42:34.608506+01:00"

 -- Function: 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.

Other Features
**************

Reader Macros
=============

 -- Function: enable-read-macros

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

Support for non-Gregorian Calendars
===================================

 -- Function: astronomical-julian-date timestamp

     Returns the julian date of the date portion of TIMESTAMP.

 -- Function: astronomical-julian-date timestamp

     Returns the modified julian date of the date portion of TIMESTAMP.

References
**********

  * [NaggumPaper] Erik Naggum. The Long Painful History of Time http://naggum.no/lugm-time.html, 1999.