%% Insert a %
%a Abbreviated weekday name
%A Full weekday name
%b Abbreviated month name
%B Full month name
%d Day of month (01 31)
%D Date as %m/%d/%y
%e Day of month (1 31), no leading zeros
%h Abbreviated month name
%H Hour (00 23)
%I Hour (00 12)
%j Day number of year (001 366)
%m Month number (01 12)
%M Minute (00 59)
%n Insert a new line
%p AM or PM
%r Time as %I:%M:%S %p
%R Time as %H:%M
%S Seconds (00 59)
%t Insert a tab
%T Time as %H:%M:%S
%U Week number of year (01 52), Sunday is the first day of the week
%w Weekday number (Sunday = 0)
%W Week number of year (01 52), Monday is the first day of the week
%x Local specific date format
%X Local specific time format
%y Year within century (00 99)
%Y Year as ccyy (for example, 1990)
%Z Time zone name
If format is not specified, ``%a %b %d %H:%M:%S %Z %Y'' is used. If GMT is specified, the time will be formatted as Greenwich Mean Time. If the argument is not specified or is empty, then the local timezone will be used as defined by the TIMEZONE environment variable.