Automatically add custom field data based on order

hi,

we're selling magazines on our website. we offer subscriptions such as 3 month, 6 month and 12 month.

i've added an extra field to the registration form on the front end that asks users to select which issue of the magazine they wish to start their subscription from.

i've also added a field to the back end of the site which allows us to enter an end date for their subscription.

is it possible to automate the process of selecting an end date? is it possible to add some code somewhere that checks the product type and the value of the start date and then automatically works out the end date for the subscription and adds this information to the database?

thanks