Hi,
if you are using MySQL, and you query the date of subscription from a table, you can use the date / time functions of Mysql,
http://www.mysql.com/doc/en/Date_and_time_functions.html
Here you have the option to add an interval in sec/min/hour/day/month/year to the selected date.
Else you could certainly use the mktime function of PhP to convert to unix time, and do calculations (For PhP without MySql there should eb an easier way, but I am unaware of it. Do a search on PhPbuilder. com! )
J.