More granular time/date macros

It would be very helpful to me to have more granular time and date macro capabilities. The pre-formatted date/time macros are available, but it would be helpful to be able to specify elements independently. Specfically, the following:

%H -- hour (00..23)
%M -- minute (00..59)
%S -- second (00..60)
%y -- last two digits of year (00..99)
%Y -- year
%m -- month (01..12)
%d -- day of month (e.g., 01)

Now, if I want a timestamp formatted as, say:
090420_122539
I can create the macro:
%m%d%y_%H%M%S