Well, the usual arrangement for a backup system is to do a rolling backup of eg all files lees than x days old, hence the section I referred you to.
That way, no files get missed out.
If you want the user to be able to specify a particular date or date range, you're going to have to read up on the date cmd http://linux.die.net/man/1/date
and read those links above to figure how to do date calcs, then convert to n days ago to be able to use find to grab them.
I guess it would be a good little programming project; its do-able but sufficiently non-trivial to be interesting and a good learning experience.