• a
  The abbreviated weekday name according to the current locale. Use format_time/4 for POSIX locale.
• A
  The full weekday name according to the current locale. Use format_time/4 for POSIX locale.
• b
  The abbreviated month name according to the current locale. Use format_time/4 for POSIX locale.
• B
  The full month name according to the current locale. Use format_time/4 for POSIX locale.
• c
  The preferred date and time representation for the current locale.
• C
  The century number (year/100) as a 2-digit integer.
• d
  The day of the month as a decimal number (range 01 to 31).
• D
  Equivalent to %m/%d/%y. (For Americans only. Americans should note that in other countries %d/%m/%y is rather common. This means that in an international context this format is ambiguous and should not be used.)
• e
  Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space.
• E
  Modifier. Not implemented.
• f
  Number of microseconds. The f can be prefixed by an integer to print the desired number of digits. E.g., %3f prints milliseconds. This format is not covered by any standard, but available with different format specifiers in various incarnations of the strftime() function.
• F
  Equivalent to %Y-%m-%d (the ISO 8601 date format).
• g
  Like %G, but without century, i.e., with a 2-digit year (00-99).
• G
  The ISO 8601 year with century as a decimal number. The 4-digit year corresponding to the ISO week number (see %V). This has the same format and value as %y, except that if the ISO week number belongs to the previous or next year, that year is used instead.
• V
  The ISO 8601:1988 week number of the current year as a decimal number, range 01 to 53, where week 1 is the first week that has at least 4 days in the current year, and with Monday as the first day of the week. See also %U and %W.
• h
  Equivalent to %b.
• H
  The hour as a decimal number using a 24-hour clock (range 00 to 23).
• I
  The hour as a decimal number using a 12-hour clock (range 01 to 12).
• j
  The day of the year as a decimal number (range 001 to 366).
• k
  The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank. (See also %H.)
• l
  The hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank. (See also %I.)
• m
  The month as a decimal number (range 01 to 12).
• M
  The minute as a decimal number (range 00 to 59).
• n
  A newline character.
• O
  Modifier to select locale-specific output. Not implemented.
• p
  Either‘AM' or‘PM' according to the given time value, or the corresponding strings for the current locale. Noon is treated as‘pm' and midnight as‘am'.^{150Despite the above claim, some locales yield am or pm in lower case.}
• P
  Like %p but in lowercase:‘am' or‘pm' or a corresponding string for the current locale.
• r
  The time in a.m. or p.m. notation. In the POSIX locale this is equivalent to‘%I:%M:%S %p'.
• R
  The time in 24-hour notation (%H:%M). For a version including the seconds, see %T below.
• s
  The number of seconds since the Epoch, i.e., since 1970-01-01 00:00:00 UTC.
• S
  The second as a decimal number (range 00 to 60). (The range is up to 60 to allow for occasional leap seconds.)
• t
  A tab character.
• T
  The time in 24-hour notation (%H:%M:%S).
• u
  The day of the week as a decimal, range 1 to 7, Monday being 1. See also %w.
• U
  The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday as the first day of week 01. See also %V and %W.
• w
  The day of the week as a decimal, range 0 to 6, Sunday being 0. See also %u.
• W
  The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday as the first day of week 01.
• x
  The preferred date representation for the current locale without the time.
• X
  The preferred time representation for the current locale without the date.
• y
  The year as a decimal number without a century (range 00 to 99).
• Y
  The year as a decimal number including the century.
• z
  The timezone as hour offset from GMT using the format HHmm. Required to emit RFC822-conforming dates (using ’%a, %d %b %Y %T %z'). Our implementation supports %:z, which modifies the output to HH:mm as required by XML-Schema. Note that both notations are valid in ISO 8601. The sequence %:z is compatible to the GNU date(1) command.
• Z
  The timezone or name or abbreviation.
• +
  The date and time in date(1) format.
• %
  A literal‘%’character.

The table below gives some format strings for popular time representations. RFC1123 is used by HTTP. The full implementation of http_timestamp/2 as available from library(http/http_header) is here.

http_timestamp(Time, Atom) :-
        stamp_date_time(Time, Date, 'UTC'),
        format_time(atom(Atom),
                    '%a, %d %b %Y %T GMT',
                    Date, posix).