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Time­Advanced­Cowan

cowan
2010-11-22 14:41:02
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Date and time arithmetic

This is a WG2 proposal for date and time arithmetic, loosely based on Java's JodaTime 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 a rational number representing a particular millisecond (or fraction thereof) 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-millisecond 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 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 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 chronology, returns 0. When chronology was returned from chronology-with-time-zone, returns the time zone specified to that procedure.

(chronology-standard-time-zone chronology)

Returns the standard time zone offset in minutes from UTC corresponding to this chronology.

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 millisecond or better. 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.

(instant->date instance chronology)

Constructs and returns a date object using chronology and corresponding to instant.

(date->instant date)

Return the instant corresponding to date, provided there is enough information in the fields of date to uniquely determine it.

(date-field date fieldname)

Returns the value of the field named fieldname 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 based on date, but with the field named fieldname incremented by increment. This may cause other fields to change as well. An error is signaled if the field is unknown or the value is out of range, or if an appropriate new date object cannot be constructed due to lack of information in date.

(date-chronology date)

Returns the chronology associated with date.

(date-field-maximum date fieldname)

Returns the maximum legal value of the field named fieldname in the chronology associated with date. This value is not necessarily the same for all instants.

(date-field-minimum date fieldname)

Returns the minimum legal value of the field named fieldname in the chronology associated with date. This value is not necessarily the same for all instants.

(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.

ISO/Gregorian/Julian date fields

Unless otherwise noted, the values of these fields are integers. These fields may be relevant to other chronologies as well.

century
The absolute century of the date. In the ISO chronology, the century of 1965 C.E. is 19 and the century of 43 B.C.E is 0; in the Gregorian and Julian calendars, they are 20 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 calendars, 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. daylight 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 no eras. In the Gregorian and Julian chronologies, C.E. (or A.D.) = 1, 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.
ms-of-day
The millisecond of the date's day, in the range 0-86399999
ms-of-second
The millisecond of the date's second, in the range 0-999. This value need not be an integer.
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, in the range 0-86399.
second-of-minute
The second of the date's minute, in the range 0-59. Note that leap seconds aren't represented.
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 calendars, the range is 1-100.
year-of-era
The year of the date's era. In the Gregorian and Julian calendars, the value cannot be 0.