vb6 w/database: dated billing

vb6 with database program that charges varying monthly rents all due on the 1st of each month::
i'm at a loss how to keep the amount owed correct if the user changes system date(add/deduct rent as the date changes) and most importantly, how to add the rent on the 1st of the month if the program is not running that day but runs the next day or so??? i tried running in startup but kept having problems.
any help would be great or even a tutorial anyone can point me to...THANKS