The following is copied from the UNIX man page for the strftime call.
The formats can be used to change date formats
The abbreviated weekday name according to the cur-
rent locale.
%A The full weekday name according to the current
locale.
%b The abbreviated month name according to the current
locale.
%B The full month name according to the current
locale.
%c The preferred date and time representation for the
current locale.
%C The century number (year/100) as a 2-digit integer.
(SU)
%d The day of the month as a decimal number (range 01
to 31).
%D Equivalent to %m/%d/%y. (Yecch - for Americans
only. Americans should note that in other coun-
tries %d/%m/%y is rather common. This means that in
international context this format is ambiguous and
should not be used.) (SU)
%e Like %d, the day of the month as a decimal number,
but a leading zero is replaced by a space. (SU)
%E Modifier: use alternative format, see below. (SU)
%G The ISO 8601 year with century as a decimal number.
The 4-digit year corresponding to the ISO week num-
ber (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. (TZ)
%g Like %G, but without century, i.e., with a 2-digit
year (00-99). (TZ)
%h Equivalent to %b. (SU)
%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.) (TZ)
%l The hour (12-hour clock) as a decimal number (range
1 to 12); single digits are preceded by a blank.
i %a The abbreviated weekday name according to the cur-
rent locale.
%A The full weekday name according to the current
locale.
%b The abbreviated month name according to the current
locale.
%B The full month name according to the current
locale.
%c The preferred date and time representation for the
current locale.
%C The century number (year/100) as a 2-digit integer.
(SU)
%d The day of the month as a decimal number (range 01
to 31).
%D Equivalent to %m/%d/%y. (Yecch - for Americans
only. Americans should note that in other coun-
tries %d/%m/%y is rather common. This means that in
international context this format is ambiguous and
should not be used.) (SU)
%e Like %d, the day of the month as a decimal number,
but a leading zero is replaced by a space. (SU)
%E Modifier: use alternative format, see below. (SU)
%G The ISO 8601 year with century as a decimal number.
The 4-digit year corresponding to the ISO week num-
ber (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. (TZ)
%g Like %G, but without century, i.e., with a 2-digit
year (00-99). (TZ)
%h Equivalent to %b. (SU)
%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.) (TZ)
%l The hour (12-hour clock) as a decimal number (range
1 to 12); single digits are preceded by a blank.s a decimal number (range 01 to 12).
%M The minute as a decimal number (range 00 to 59).
%n A newline character. (SU)
%O Modifier: use alternative format, see below. (SU)
%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'.
%P Like %p but in lowercase: `am' or `pm' or a corre-
sponding string for the current locale. (GNU)
%r The time in a.m. or p.m. notation. In the POSIX
locale this is equivalent to `%I:%M:%S %p'. (SU)
%R The time in 24-hour notation (%H:%M). (SU) 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. (TZ)
%S The second as a decimal number (range 00 to 61).
%t A tab character. (SU)
%T The time in 24-hour notation (%H:%M:%S). (SU)
%u The day of the week as a decimal, range 1 to 7,
Monday being 1. See also %w. (SU)
%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.
%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. (SU)
%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 time-zone as hour offset from GMT. Required to emit RFC822-conformant dates (using "%a, %d %b %Y %H:%M:%S %z"). (GNU)
%Z The time zone or name or abbreviation.
%+ The date and time in date(1) format. (TZ)
%% A literal `%' character.