SWI-Prolog -- date_time

Availability:built-in

date_time_stamp(+DateTime, -TimeStamp)

Compute the timestamp from a date/9 term. Values for month, day, hour, minute or second need not be normalized. This flexibility allows for easy computation of the time at any given number of these units from a given timestamp. Normalization can be achieved following this call with stamp_date_time/3. This example computes the date 200 days after 2006-07-14:

?- date_time_stamp(date(2006,7,214,0,0,0,0,-,-), Stamp),
   stamp_date_time(Stamp, D, 0),
   date_time_value(date, D, Date).
Date = date(2007, 1, 30)

When computing a time stamp from a local time specification, the UTC offset (arg 7), TZ (arg 8) and DST (arg 9) argument may be left unbound and are unified with the proper information. The example below, executed in Amsterdam, illustrates this behaviour. On the 25th of March at 01:00, DST does not apply. At 02.00, the clock is advanced by one hour and thus both 02:00 and 03:00 represent the same time stamp.

1 ?- date_time_stamp(date(2012,3,25,1,0,0,UTCOff,TZ,DST),
                     Stamp).
UTCOff = -3600,
TZ = 'CET',
DST = false,
Stamp = 1332633600.0.

2 ?- date_time_stamp(date(2012,3,25,2,0,0,UTCOff,TZ,DST),
                     Stamp).
UTCOff = -7200,
TZ = 'CEST',
DST = true,
Stamp = 1332637200.0.

3 ?- date_time_stamp(date(2012,3,25,3,0,0,UTCOff,TZ,DST),
                     Stamp).
UTCOff = -7200,
TZ = 'CEST',
DST = true,
Stamp = 1332637200.0.

Note that DST and offset calculation are based on the POSIX function mktime(). If mktime() returns an error, a representation_error dst is generated.