Title

SRFI 19: Time Data Types and Procedures

Author

Will Fitzgerald

Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. You can access the discussion via the archive of the mailing list.

Abstract

Points in time are represented a the number of seconds (with nanosecond precision) since "the epoch," a zero point in time. Several standard variants are defined, including UTC (universal coordinated time), TAI (international atomic time), and monotonic time. A point in time can also be represented as a Julian Day or Modified Julian Day number. Time durations, including time spent in a process or thread, are defined. Conversion routines are provided. The procedure CURRENT-TIME queries the current time in a specified variant, with a system-dependent resolution. Procedures for time arithmetic and time comparisons are also provided.

A date is a representation of a point in time in the Gregorian calendar, a 24 hour clock (with nanosecond precision) and a time zone offset from UTC. Procedures for converting between time and dates are provided, as well as for reading and writing string representations of dates.

Issues

[None currently]

Rationale

R5RS Scheme does not provide standard data types for time. This SRFI addresses this lack by specifying data types for time and associated procedures.

Specification

A Time object, which is distinct from all existing types, defines a point in time or a time duration in some standard time system. The standard time systems are:

Implementations are required to implement UTC, monotonic time, CPU time in current process, and time duration. Implementations are allowed to create extensions (for example, amount of time spent in garbage collection).

A time object consists of three components:

A Date object, which is distinct from all existing types, represents a point in time as represented by the Gregorian calendar as well as by a time zone. Dates are immutable. A date consists of the following components:

A Julian Day represents a point in time as a real number of days since -4714-11-24T12:00:00Z (November 24, -4714 at noon, UTC).

A Modified Julian Day represents a point in time as a real number of days since 1858-11-17T00:00:00Z (November 17, 1858 at midnight, UTC).

Constants

The following constants are required:

time-duration
Symbol representing Time duration.
time-monotonic
Symbol representing monotonic time.
time-process
Symbol representing time spent in current process.
time-tai
Symbol representing TAI time.
time-thread
Symbol representing time spent in current thread.
time-utc
Symbol representting UTC time.

Current time and clock resolution

The following procedures are required:

current-date [tz-offset] -> date
Date corresponding to the current UTC time.
current-julian-day -> jdn
Current Julian Day.
current-modified-julian-day -> mjdn
Current Modified Julian Day.
current-time [time-type] -> time
Current time, of type time-type system, which defaults to TIME-UTC.
time-resolution [time-type] -> integer
Clock resolution, in nanoseconds, of the system clock of type type time-type system, which defaults to TIME-UTC.

Time object and accessors

The following procedures are required:

make-time type nanosecond second -> time
Creates a time object.
time? object -> boolean
#t if object is a time object, otherwise, #f.
time-type time -> time-type
Time type.
time-nanosecond time -> integer
Time nanosecond.
time-second time -> integer
Time second.
set-time-type! time time-type
Changes time type. Note: This changes the semantics of the time object. To convert a time to another system of representation, use one of the conversion procedures.
set-time-nanosecond! time integer
Changes time nanosecond.
set-time-second! time integer
Changes time second.
copy-time time1 -> time2
Creates a new time object, with the same time type, nanosecond, and second as time1.

Time comparison procedures

All of the time comparison procedures require the time objects to be of the same type. It is an error to use these procedures on time objects of different types. For the point-in-time measurements (e.g., TIME-TAI and TIME-UTC), the semantics are described in plain text. For durations, (e.g., TIME-DURATION, TIME-CPU, the semantics are described in parentheses.

The following procedures are required:

time<=? time1 time2 -> boolean
#t if time1 is before or at (less than or equal to) time2, #f otherwise.
time<? time1 time2 -> boolean
#t if time1 is before (less than) time2, #f otherwise.
time=? time1 time2 -> boolean
#t if time1 at (equal) time2, #f otherwise.
time>=? time1 time2 -> boolean
#t if time1 is at or after (greater than or equal to) time2, #f otherwise.
time>? time1 time2 -> boolean
#t if time1 is after (greater than) time2, #f otherwise.

Time arithmetic procedures

The following procedures are required.

time-difference time1 time2 -> time-duration
The TIME-DURATION between time1 and time2. It is an error if time1 and time2 are of different time types. A new time object is created.
time-difference! time1 time2 -> time-duration
The TIME-DURATION between time1 and time2. It is an error if time1 and time2 are of different time types. Time1 may be used to create the resulting TIME-DURATION object.
add-duration time1 time-duration -> time
The time resulting from adding time-duration to time1, which is a time object of the same time type as time1. A new time object is created.
add-duration! time1 time-duration -> time
The time resulting from adding time-duration to time1, which is a time object of the same time type as time1. Time1 may used to create the resulting time object.
subtract-duration time1 time-duration -> time
The time resulting from subtracting time-duration to time1, which is a time object of the same time type as time1. A new time object is created.
subtract-duration! time1 time-duration -> time
The time resulting from subtracting time-duration to time1, which is a time object of the same time type as time1. Time1 may used to create the resulting time object.

Date object and accessors

Date objects are immutable once created. The following procedures are required.

make-date nanosecond second minute hour day month year zone-offset -> date
Creates a date object.
date? date -> boolean
#t if object is a time object, otherwise, #f.
date-nanosecond date -> integer
Date nanosecond.
date-second date -> integer
Date second.
date-minute date -> integer
Date minute.
date-hour date -> integer
Date hour.
date-day date -> integer
Date day.
date-month date -> integer
Date month.
date-year date -> integer
Date year.
date-zone-offset date -> integer
Date time zone offset.
date-year-day date -> integer
The ordinal day of the year of this date. January 1 is 1, etc.
date-week-day date -> integer
The day of the week of this date, where Sunday=0, Monday=1, etc.
date-week-number date day-of-week-starting-week -> integer
The ordinal week of the year which holds this date, ignoring a first partial week. 'Day-of-week-starting-week' is the integer corresponding to the day of the week which is to be considered the first day of the week (Sunday=0, Monday=1, etc.).

Time/Date/Julian Day/Modified Julian Day Converters

The following conversion procedures are required.
date->julian-day date -> jd
Convert date to Julian Day.
date->modified-julian-day date -> mjd
Convert date to Modified Julian Day.
date->time-monotonic date -> time-monotonic
Convert date to monotonic time.
date->time-tai date -> time-tai
Convert date to TAI time.
date->time-utc date -> time-utc
Convert date to UTC time.
julian-day->date jd [tz-offset] -> date
Convert Julian Day to date, , using time zone offset, which defaults to the local time zone.
julian-day->time-monotonic jd -> time-monotonic
Convert Julian Day to monotonic time.
julian-day->time-tai jd -> time-tai
Convert Julian Day to TAI time.
julian-day->time-utc jd -> time-utc
Convert Julian Day to UTC time.
modified-julian-day->date mjd [tz-offset] -> date
Convert Modified Julian Day to date, using time zone offset, which defaults to the local time zone.
modified-julian-day->time-monotonic mjd -> time-monotonic
Convert Modified Julian Day to monotonic time.
modified-julian-day->time-tai mjd -> time-tai
Convert Modified Julian Day to TAI time.
modified-julian-day->time-utc mjd -> time-utc
Convert Modified Julian Day to UTC time.
time-monotonic->date time-monotonic [tz-offset] -> date
Convert monotonic time to date, using time zone offset, which defaults to the local time zone.
time-monotonic->julian-day time-monotonic -> jd
Convert monotonic time to Julian Day.
time-monotonic->modified-julian-day time-monotonic -> mjd
Convert monotonic time to Modified Julian Day.
time-monotonic->time-tai time-monotonic -> time-tai
Convert monotonic time to TAI time.
time-monotonic->time-tai! time-monotonic -> time-tai
Convert monotonic time to TAI time. The time structure may be reused.
time-monotonic->time-utc time-monotonic -> time-utc
Convert monotonic time to UTC time.
time-monotonic->time-utc! time-monotonic -> time-utc
Convert monotonic time to UTC time. The time structure may be reused.
time-tai->date time-tai [tz-offset] -> date
Convert TAI time to date, using time zone offset, which defaults to the local time zone.
time-tai->julian-day time-tai -> jd
Convert TAI time to Julian Day.
time-tai->modified-julian-day time-tai -> mjd
Convert TAI time to Modified Julian Day.
time-tai->time-monotonic time-tai -> time-monotonic
Convert TAI time to monotonic time.
time-tai->time-monotonic! time-tai -> time-monotonic
Convert TAI time to monotonic time. The time structure may be reused.
time-tai->time-utc time-tai -> time-utc
Convert TAI time to monotonic time.
time-tai->time-utc! time-tai -> time-utc
Convert TAI time to monotonic time. The time structure may be reused.
time-utc->date time-utc [tz-offset] -> time-utc
Convert UTC time to date, using time zone offset, which defaults to the local time zone.
time-utc->julian-day time-utc -> jd
Convert UTC time to Julian Day
time-utc->modified-julian-day time-utc -> mjd
Convert UTC time to Modified Julian Day.
time-utc->time-monotonic time-utc -> time-monotonic
Convert UTC time to monotonic time.
time-utc->time-monotonic! time-utc -> time-monotonic
Convert UTC time to monotonic time. The time structure may be reused.
time-utc->time-tai time-utc -> time-tai
Convert UTC time to TAI time.
time-utc->time-tai! time-utc -> time-tai
Convert UTC time to TAI time. The time structure may be reused.

Date to String/String to Date Converters

These procedures provide conversion to and from strings. They are required. The specification below describes a 'locale;' the specification of locales is beyond this SRFI.
date->string date [format-string] -> string
Converts a date to a string, using the format string. The format string is copied as is; except escape characters (indicated by the tilde) are replaced with specific conversions. Table 1 lists the required conversion specifiers; implementations are free to extend this list.
string->date input-string template-string -> date
Converts an input string to a date, using the template string. The input string must match the template sting as is; except escape characters (indicate by the tilde) indicate special converters which (1) move to the next character in the input string fulfilling a criterion; (2) read a value, and (3) act on this value in some way. Table 2 lists the required converters; implementations are free to extend this list.

PLT-specific extensions

The ~? wildcard is specific to the PLT implementation of string->date: it parses 1 and 2 digit years like ~y and 3 and 4 digit years like ~Y.


ChConversion

~~a literal ~
~alocale's abbreviated weekday name (Sun...Sat)
~Alocale's full weekday name (Sunday...Saturday)
~blocale's abbreviate month name (Jan...Dec)
~Blocale's full month day (January...December)
~clocale's date and time (e.g., "Fri Jul 14 20:28:42-0400 2000")
~dday of month, zero padded (01...31)
~Ddate (mm/dd/yy)
~eday of month, blank padded ( 1...31)
~fseconds+fractional seconds, using locale's decimal separator (e.g. 5.2).
~hsame as ~b
~Hhour, zero padded, 24-hour clock (00...23)
~Ihour, zero padded, 12-hour clock (01...12)
~jday of year, zero padded
~khour, blank padded, 24-hour clock (00...23)
~lhour, blank padded, 12-hour clock (01...12)
~mmonth, zero padded (01...12)
~Mminute, zero padded (00...59)
~nnew line
~Nnanosecond, zero padded
~plocale's AM or PM
~rtime, 12 hour clock, same as "~I:~M:~S ~p"
~snumber of full seconds since "the epoch" (in UTC)
~Ssecond, zero padded (00...60)
~thorizontal tab
~Ttime, 24 hour clock, same as "~H:~M:~S"
~Uweek number of year with Sunday as first day of week (00...53)
~Vweek number of year with Monday as first day of week (01...52)
~w day of week (0...6)
~Wweek number of year with Monday as first day of week (01...52)
~xweek number of year with Monday as first day of week (00...53)
~Xlocale's date representation, for example: "07/31/00"
~ylast two digits of year (00...99)
~Yyear
~ztime zone in RFC-822 style
~Zsymbol time zone (not-implemented)
~1ISO-8601 year-month-day format
~2ISO-8601 hour-minute-second-timezone format
~3ISO-8601 hour-minute-second format
~4ISO-8601 year-month-day-hour-minute-second-timezone format
~5ISO-8601 year-month-day-hour-minute-second format

Table 1: DATE->STRING conversion specifiers


ChSkip toReadSet

~~anyread literal ~nothing
~achar-alphabetic?abbreviated weekday in localenothing
~Achar-alphabetic?full weekday in localenothing
~bchar-alphabetic?abbreviated month name in localenothing
~Bchar-alphabetic?full month name in localenothing
~dchar-numeric?day of monthdate-day
~eanyday of month, blank paddeddate-day
~hchar-alphabetic?same as ~bnothing
~Hchar-numeric?hourdate-hour
~kanyhour, blank paddeddate-hour
~mchar-numeric?monthdate-month
~Mchar-numeric?minutedate-minute
~Schar-numeric?seconddate-second
~yany2-digit yeardate-year within 50 years
~Ychar-numeric?yeardate-year
~zanytime zonedate-zone-offset
~?char-numeric?2-digit or 4-digit year (PLT-specific extension)date-year

Table 2: STRING->DATE conversion specifiers

Implementation

This SRFI cannot be written in completely standard Scheme. In particular, there must be some system-independent method of finding the values for CURRENT-TIME. The GNU C function, gettimeofday might prove useful to implementors.

The difference between TAI and UTC is not determinate, and implementations must provide some method for getting TAI. A procedure is provided in the accompany implmentation for reading the leap second table provided by the Time Service of the US Naval Observatory (available at ftp://maia.usno.navy.mil/ser7/tai-utc.dat).

The accompanying implementation assumes SRFI 6 Basic String Ports. The accompanying implementation also assumes an error procedure. The accompanying implementation also assumes SRFI 8 RECEIVE: Binding to multiple values. which is easy to implement with the following syntax:

(define-syntax receive
  (syntax-rules ()
    ((receive formals expression body ...)
     (call-with-values (lambda () expression)
                       (lambda formals body ...)))))

Note that it contains TAI-UTC.DAT reader.

The accompanying implementation is written in MzScheme. MzScheme provides the procedure current-seconds, which returns the number of seconds (UTC) since 1970-01-01T00:00:00Z+00:00, and current-milliseconds, which is a monotonic time clock. Combining these provides an implementation of (current-time time-utc). Monontonic time, in this implementation, is the same as TAI time; differences between TAI and UTC are resolved through a leap second table. According to the International Earth Rotation Service, there will be no leap second in December, 2000. Thus, the leap second table is guaranteed to be correct through June, 2000.

Also, MzScheme (as of version 102, I believe) provides a method for returning the current time zone offset, via its SECONDS->DATE and CURRENT-DATE procedures.

MzScheme's DEFINE-STRUCT was used to define the time and date objects. SRFI 9, Defining Record Types, could be used instead.

Procedures meant to be used internally have names beginning with TM:. Locale-related constants and procedures have locale in their name; if a 'locale' SRFI is ever written, it might be good to use that code instead.

From this, the rest of the implementation was built.

There is also a test suite.

Acknowledgements

Claus T√łndering's Frequently Asked Questions about calendars was a very useful resource. The implementation of Julian Day, Modified Julian Day, and Year Day comes from his FAQ. Markus Kuhn has a useful description of the ISO Standard 8601 for Date/Time notation; The W3 Consortium also has a useful Note.

Mike Sperber, Marc Feely, Dave Mason, and "Prfnoff" all made useful comments on previous versions of this draft. Thanks to Shriram Krishnamurthi for his editing help.

The DATE->STRING procedure uses a format string, based on GNU C's date procedure, as well as scsh's FORMAT-DATE procedure.

Copyright

Copyright (C) Neodesic Corporation (2000). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.


Editor: Shriram Krishnamurthi
Last modified by the author:
(display (date->string (current-date 0) "~4")): 2004-03-15T02:21:15Z