create string output with custom format

[bchurchill]

Many date/time libraries will have a format() method that takes a string containing escape sequences, and then format the date according to that string. We might want, for example, escapes like this for this library:

%d  day of the month
%m  month
%y  year
%w  name of weekday
%H  hour
%M  minute
%S  second