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== Date and time arithmetic ==

This is a WG2 proposal for date and time arithmetic, loosely based on Java's [http://joda-time.sourceforge.net Joda Time] functions. It's possible to implement SRFI 19 on top of it, but it provides much more flexibility.

== Instants ==

For the purposes of this proposal, an ''instant'' is an integer representing a particular second of the Posix epoch, which began on 00:00:00 on 1 January 1970, Coordinated Universal Time, but excludes all leap seconds. See TimeCowan for the `current-posix-second` procedure, which returns the current instant.

== Chronologies ==

A ''chronology'' is a disjoint and immutable object that describes a particular calendar, such as the ISO, Gregorian, Julian, Jewish, Islamic, Persian, French Revolutionary, Maya, Chinese, Buddhist, Coptic, or Ethiopic calendar. Implementations MUST support the ISO chronology, and MAY support any of the other calendars mentioned here, or indeed any calendar not mentioned here.

Chronologies also incorporate the concept of time zone. An implementation MUST support the UTC (Universal Coordinated Time) timezone and any time zones expressible as a fixed offset in minutes from UTC. It SHOULD support the historical time zones of the [http://en.wikipedia.org/wiki/Tz_database tz database].

== Chronology procedures ==

`(default-chronology)`

Returns the default system-dependent chronology, which may include time zone information.

`(chronology `''symbol''`)`

Return a chronology named by ''symbol''. The ISO chronology is named `iso`. If provided, the Gregorian chronology is named `gregorian` and the Julian chronology is named `julian`. The names of other chronologies are system-dependent. The chronologies returned by this procedure are in the UTC time zone.

`(chronology-names)`

Returns a list of the symbols naming the chronologies provided by the implementation. It is an error to modify this list.

`(chronology-name `''chronology''`)`

Returns the symbol that names ''chronology''.

`(chronology? `''obj''`)`

Returns `#t` if ''obj'' is a chronology and `#f` otherwise.

`(chronology-with-time-zone `''chronology''` `''timezone''`)`

Returns a chronology object based on ''chronology'', but with the time zone specified by ''timezone''. If ''timezone'' is an integer, it represents the number of minutes ahead of UTC. If the implementation supports the tz database, and ''timezone'' is a string containing a time zone name defined by that database, it represents the time zone with that name. Otherwise, the interpretation of ''timezone'' is implementation-dependent.

`(chronology-time-zone `''chronology''`)`

When ''chronology'' was returned from `default-chronology`, returns an implementation-dependent value. When ''chronology'' was returned from `chronology`, returns 0. When ''chronology'' was returned from `chronology-with-time-zone`, returns the time zone value (of whatever type) which was specified to that procedure.

`(make-compound-chronology `''chronology1''` `''chronology2''` `''instant''`)`

Returns a chronology that uses ''chronology2'' at all times before ''instant'', and ''chronology1'' for all times including and following ''instant''. This is useful for constructing chronologies that transition from Julian to Gregorian at a specified date.

== Date objects ==

A ''date object'' is a member of an immutable disjoint type that specifies information about a date or time with respect to a certain chronology. For example, with respect to the ISO, Gregorian, or Julian chronologies, a date may represent a specific year, a specific week of a specific year, or an instant in time precise to a second. On the other hand, a date object may contain just a particular month. Date objects have multiple numeric-valued fields such as `year` or `minute-of-day`, whose meanings and possible values are determined by the chronology.

== Date procedures ==

`(make-date `''chronology''` `''alist''`)`

Constructs and returns a date object using ''chronology''. ''Alist'' is an association list mapping symbols which are names of fields meaningful to ''chronology'' to associated numeric values. An error is signaled if the field values are invalid or inconsistent.

`(date? `''obj''`)`

Returns `#t` if ''obj'' is a date object, and `#f` otherwise.

`(date->alist `''date''`)`

Constructs and returns an alist containing the fields of ''date''. Implementations SHOULD provide computed fields as well as explicitly set ones.

`(date-field `''date''` `''fieldname''`)`

Returns the numeric value of the field named ''fieldname'' (a symbol) within ''date'', or `#f` if there is no such field. If the specified field was not provided when ''date'' was constructed, and it is possible to compute its value from the values of fields that were provided, the value is computed and returned.

`(date-update `''date''` `''fieldname''` `''value''`)`

Constructs and returns a new date object based on ''date'', but with the field named ''fieldname'' updated to ''value''. An error is signaled if the field is unknown or the value is out of range.

`(date-increment `''date''` `''fieldname''` `''increment''`)`

Constructs and returns a new date object which is later than ''date'' by
''increment'' measured in the units specified by ''fieldname'', or
earlier if ''increment'' is negative. Returns `#f` if an appropriate
date object cannot be constructed due to lack of information in ''date''
An error is signaled if ''fieldname'' is unknown.

For example, `(date-increment `''date''` 'day-of-month 7)` adds seven days to ''date''
(which is typically the same as one week, but may be different in some chronologies).

`(date-chronology `''date''`)`

Returns the chronology associated with ''date''.

`(date-field-maximum `''date''` `''fieldname''`)`

`(date-field-minimum `''date''` `''fieldname''`)`

Returns the maximum or minimum legal value of the field named ''fieldname'' in the chronology associated with ''date''. This value is not necessarily the same for all instants; for example, 28 is the maximum value of `day-of-month` if `month` has the value 2 (February). Returns `#f` if the value cannot be determined (there is no maximum or minimum year in the ISO chronology, for example).

`(date-round `''date''` `''fieldname''`)`

`(date-ceiling `''date''` `''fieldname''`)`

`(date-floor `''date''` `''fieldname''`)`

`(date-truncate `''date''` `''fieldname''`)`

Constructs and returns a new date object which is the same as ''date'', but adjusted to the nearest integral value of ''fieldname'' using the `round`, `ceiling`, `floor`, or `truncate` functions. This may cause other fields to change their values as well.

== Date fields ==

Unless otherwise noted, the values of these fields MUST be either exact integers or `#f` if they were not explicitly set and cannot be calculated from the given information. These fields are specified for the ISO, Gregorian, and Julian chronologies, but they may be relevant to other chronologies as well. For interoperability, every chronology must support the `instant` field.

`century`::
The absolute century of the date. In the ISO chronology, the century of 1965 C.E. is 19, the century of 70 C.E. is 0, and the century of 43 B.C.E is -1. In the Gregorian and Julian chronologies, they are 20, 1, and -1 respectively, and there is no century 0.

`century-of-era`::
The century of the date's era.

`clock-hour-of-day`::
The hour of the date's day in the range 1-24.

`clock-hour-of-half-day`::
The hour of the date's half-day, in the range 1-12.

`day-of-month`::
The day of the date's month, in the range 1-31 (or less in some months).

`day-of-week`::
The day of the date's week. In the ISO chronology, Monday is day 1 and Sunday is day 7. In the Julian and Gregorian chronologies, Sunday is day 1 and Saturday is day 7.

`day-of-year`::
The day of the date's year, in the range 1-365 in non-leap years and 1-366 in leap years.

`daylight-saving-time`::
1 if daylight saving time (summer time) was in effect, or 0 if not. This field discriminates between 2:00 A.M. daylight time and 2:00 A.M. standard time on the day when daylight saving time ends in the U.S. (and the corresponding periods for other daylight saving time regimes).

`era`::
The date's era. The ISO chronology has just one era. In the Gregorian and Julian chronologies, C.E. (or A.D.) = 1, and B.C.E. (or B.C.) = 0. By convention, the era containing the Posix epoch is era 1.

`half-day-of-day`::
The half-day (A.M. or P.M.) of this day. A.M. = 0, P.M. = 1.

`hour-of-day`::
The hour of the date's day, in the range 0-23.

`hour-of-half-day`::
The hour of the date's half-day, in the range 0-11.

`instant`::
The instant corresponding to this date.

`local-time-offset`::
The local time zone offset (standard or daylight saving, as the case may be) in minutes ahead of UTC.

`minute-of-day`::
The minute of the date's day, in the range 0-1439.

`minute-of-hour`::
The minute of the date's hour, in the range 0-59.

`month-of-year`::
The month of the date's year, in the range 1-12.

`second-of-day`::
The second of the date's day. This value MAY be a non-integer; it MUST be greater than or equal to 0 and less than 86400.

`second-of-minute`::
The second of the date's minute. This value MAY be a non-integer; it MUST be greater than or equal to 0 and less than 60.

`standard-time-offset`::
The standard time zone offset in minutes ahead of UTC.

`week-of-week-year`::
The week of the date's week-year, in the range 1-53.

`week-year`::
The date's week-year, which begins on the week that has at least 4 days in the chronological year.

`week-year-of-century`::
The week-year of the date's century. The range is the same as for `year-of-century`.

`year`::
The absolute year of the date.

`year-of-century`::
The year of the date's century. In the ISO chronology, the range is 0-99. In the Gregorian and Julian chronologies, the range is 1-100.

`year-of-era`::
The year of the date's era. In the Gregorian and Julian chronologies, the value cannot be 0.

time

2010-12-19 13:43:49