Sets the format for date / time labels. To enable date / time
format labels see the options to
(const char *, fmt)
This string is passed directly to the system strftime. See
the system documentation for a full list of conversion
specifications for your system. All conversion
specifications take the form of a '%' character followed by
further conversion specification character. All other text
is printed as-is. Common options include:
%c: The preferred date and time
representation for the current locale.
%d: The day of the month as a
%H: The hour as a decimal number
using a 24-hour clock.
%j: The day of the year as a
%m: The month as a decimal number.
%M: The minute as a decimal number.
%S: The second as a decimal number.
%y: The year as a decimal number
without a century.
%Y: The year as a decimal number
including a century.
This function is used in example 29.