strftime() format codes

%a  Abbreviated weekday name.
%A  Full weekday name.
%b  Abbreviated month name.
%B  Full month name.
%c  Date and time representation (eg. Fri Jun 24 13:55:16 2016).
%C  Year divided by 100 (eg. 20 for 2016).
%d  Day of the month, zero-padded.
%D  MM/DD/YY date (equivalent to %m/%d/%y).
%e  Day of the month, space-padded.
%F  YYYY-MM-DD date (equivalent to %Y-%m-%d).
%g  Week-based year*, last two digits (eg. 16 for 2016).
%G  Week-based year.
%h  Abbreviated month name (same as %b).
%H  Hour in 24-hour format, zero-padded.
%I  Hour in 12-hour format, zero-padded.
%j  Day of the year.
%m  Month as a number.
%M  Minute.
%n  New-line character.
%p  AM or PM.
%r  12-hour clock time (eg. 02:34:55 pm).
%R  HH:MM 24-hour time (equivalent to %H:%M).
%S  Second.
%t  Horizontal tab character.
%T  ISO 8601** time format (HH:MM:SS) (equivalent to %H:%M:%S).
%u  ISO 8601 weekday as number, starting with Monday as 1.
%U  Week number with first Sunday as first day of first week.
%V  ISO 8601 week number.
%w  Weekday as number, starting with Sunday as 0.
%W  Week number with first Monday as first day of first week.
%x  Date representation (locale dependent).
%X  Time representation (locale dependent).
%y  Last two digits of year.
%Y  Year.
%z  ISO 8601 offset from UTC timezone (1 minute = 1, 1 hour = 100). No
    characters if timezone cannot be determined.
%Z  Timezone name or abbreviation. No characters if timezone cannot be
    determined.

 * The "week-based year" is a year defined to have either 52 or 53 full weeks.
   The 53rd week is called a "leap week". See [1].
** See [2].

References
1. [ISO Week date](https://en.wikipedia.org/wiki/ISO_week_date).
2. [ISO 8601](http://www.iso.org/iso/home/standards/iso8601.htm).

Bibliography
1. [strftime](http://www.cplusplus.com/reference/ctime/strftime/)