From the oDC Source (thanks Opera for this file)

The time format can contain any text, including the following commands:
%a = Abbreviated weekday name
%A = Full weekday name
%b = Abbreviated month name
%B = Full month name
%c = Date and time representation appropriate for locale
%d = Day of month as decimal number (01 – 31)
%H = Hour in 24-hour format (00 – 23)
%I = Hour in 12-hour format (01 – 12)
%j = Day of year as decimal number (001 – 366)
%m = Month as decimal number (01 – 12)
%M = Minute as decimal number (00 – 59)
%p = Current locale's A.M./P.M. indicator for 12-hour clock
%S = Second as decimal number (00 – 59)
%U = Week of year as decimal number, with Sunday as first day of week (00 – 53)
%w = Weekday as decimal number (0 – 6; Sunday is 0)
%W = Week of year as decimal number, with Monday as first day of week (00 – 53)
%x = Date representation for current locale
X = Time representation for current locale
%y = Year without century, as decimal number (00 – 99)
%Y = Year with century, as decimal number
%z or %Z = Either the time-zone name or time zone abbreviation,
depending on registry settings; no characters if time zone is unknown
%% = Percent sign"