Apache's SSI documentation helpfully doesn't explain how to use their <!--#config timefmt=""--> directive to modify the date format. Instead they refer you to the documentation from the C runtime they use to format the date. Really helpful to people who just want to muck with HTML, there.
So, here's the basics. The string you specify can contain "special" sequences that start with the percent character (%) that indicate the start of a time field. Each instance of these sequences gets replaced as follows:
%a- A three-letter abbreviation for the day of the week.
%A- The full name for the day of the week, one of "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", or "Saturday".
%b- A three-letter abbreviation for the month name.
%B- The full name of the month, one of "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December".
%c- A string representing the complete date and time, in the form
"%a %b %e %H:%M:%S %Y"(example "Mon Apr 01 13:13:13 1992"). %C- The century, that is, the year divided by 100 then truncated. For 4-digit years, the result is zero-padded and exactly two characters; but for other years, there may a negative sign or more digits. In this way, "
%C%y" is equivalent to "%Y". %d- The day of the month, formatted with two digits (from "01" to "31").
%D- A string representing the date, in the form "
%m/%d/%y". %e- The day of the month, formatted with leading space if single digit (from "1" to "31").
%F- A string representing the ISO 8601:2000 date format, in the form "
%Y-%m-%d". %g- The last two digits of the week-based year, see specifier %G (from "00" to "99").
%G- The week-based year. In the ISO 8601:2000 calendar, week 1 of the year includes January 4th, and begin on Mondays. Therefore, if January 1st, 2nd, or 3rd falls on a Sunday, that day and earlier belong to the last week of the previous year; and if December 29th, 30th, or 31st falls on Monday, that day and later belong to week 1 of the next year. For consistency with "
%Y", it always has at least four characters. Example: "%G" for Saturday 2nd January 1999 gives "1998", and for Tuesday 30th December 1997 gives "1998". %h- A three-letter abbreviation for the month name (synonym for "
%b"). %H- The hour (on a 24-hour clock), formatted with two digits (from "00" to "23").
%I- The hour (on a 12-hour clock), formatted with two digits (from "01" to "12").
%j- The count of days in the year, formatted with three digits (from "001" to "366").
%k- The hour (on a 24-hour clock), formatted with leading space if single digit (from "0" to "23"). (Non-POSIX extension, may not work on all Apache installs.)
%l- The hour (on a 12-hour clock), formatted with leading space if single digit (from "1" to "12"). (Non-POSIX extension, may not work on all Apache installs.)
%m- The month number, formatted with two digits (from "01" to "12").
%M- The minute, formatted with two digits (from "00" to "59").
%p- Either "AM" or "PM" as appropriate.
%r- The 12-hour time, to the second. Equivalent to "
%I:%M:%S %p". %R- The 24-hour time, to the minute. Equivalent to "
%H:%M". %S- The second, formatted with two digits (from "00" to "60"). The value 60 accounts for the occasional leap second.
%T- The 24-hour time, to the second. Equivalent to "
%H:%M:%S". %u- The weekday as a number, 1-based from Monday (from "1" to "7").
%U- The week number, where weeks start on Sunday, week 1 contains the first Sunday in a year, and earlier days are in week 0. Formatted with two digits (from "00" to "53"). See also "
%W". %V- The week number, where weeks start on Monday, week 1 contains January 4th, and earlier days are in the previous year. Formatted with two digits (from "01" to "53"). See also "
%G". %w- The weekday as a number, 0-based from Sunday (from "0" to "6").
%W- The week number, where weeks start on Monday, week 1 contains the first Monday in a year, and earlier days are in week 0. Formatted with two digits (from "00" to "53").
%x- A string representing the complete date, equivalent to "
%m/%d/%y". %X- A string representing the full time of day (hours, minutes, and seconds), equivalent to "
%H:%M:%S". %y- The last two digits of the year (from "00" to "99").
%Y- The full year, equivalent to `%C%y'. It will always have at least four characters, but may have more. The year is accurate even when tm_year added to the offset of 1900 overflows an int.
%z- The offset from UTC. The format consists of a sign (negative is west of Greewich), two characters for hour, then two characters for minutes (-hhmm or +hhmm). If tm_isdst is negative, the offset is unknown and no output is generated; if it is zero, the offset is the standard offset for the current time zone; and if it is positive, the offset is the daylight savings offset for the current timezone. The offset is determined from the TZ environment variable, as if by calling tzset().
%Z- The time zone name. If tm_isdst is negative, no output is generated. Otherwise, the time zone name is based on the TZ environment variable, as if by calling tzset().
%%- A single character, "%".