Returns a string representation of the receiver using the
receivers format. If the optionalFormatString argument is present, the
receiver's format is set to it first. Formatting is according to ANSI C
date formatting rules.
%a abbreviated weekday name (Sun, Mon, etc.)
%A full weekday name (Sunday, Monday, etc.)
%b abbreviated month name (Jan, Feb, etc.)
%B full month name (January, February, etc.)
%c full date and time string
%d day of the month as two-digit decimal integer (01-31)
%H hour as two-digit 24-hour clock decimal integer (00-23)
%I hour as two-digit 12-hour clock decimal integer (01-12)
%m month as a two-digit decimal integer (01-12)
%M minute as a two-digit decimal integer (00-59)
%p either "AM" or "PM"
%S second as a two-digit decimal integer (00-59)
%U number of week in the year as two-digit decimal integer (00-52)
with Sunday considered as first day of the week
%w weekday as one-digit decimal integer (0-6) with Sunday as 0
%W number of week in the year as two-digit decimal integer (00-52)
with Monday considered as first day of the week
%x full date string (no time); in the C locale, this is equivalent
to "%m/%d/%y".
%y year without century as two-digit decimal number (00-99)
%Y year with century as four-digit decimal number
%Z time zone name (e.g. EST);
null string if no time zone can be obtained
%% stands for '%' character in output string.