Below is an extract from the strftime documentation.
Make sure that the length of the resulting date or time string is constant and does not exceed 21 characters.
%d | the day of the month as a decimal number (01 to 31) |
%e | like %d, but a leading zero is replaced by a space |
%m | the month as a decimal number (01 to 12) |
%b | the abbreviated month name |
%Y | the year as a decimal number including the century |
%y | like %Y but without a century (00 to 99) |
%H | the hour as a number using a 24-hour clock (00 to 23) |
%k | like %H, but single digits are preceded by a blank |
%I | the hour as a number using a 12-hour clock (01 to 12) |
%l | (letter el) like %I but single digits are preceded by a blank |
%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 |
%M | the minute as a decimal number (00 to 59) |
%S | the second as a number (00 to 60) |
%% | a literal % character |
%H:%M | 13:59 |
%l:%M:%S%P | 1:59:07pm |
AUTO.
strftime documentation for more information.