Getting access to date when user select an option

Hello.
I need to know if when the user select an option, can I get the date to use it in my formula? In fact I need to know the number of month we are from June if the user decide to subscribe just for the rest of the year.
So My base price would be
base_price = number_of_month_from_June * price_determined_by_other_condition.

Ideally, can I use php and return the answer to the plugin? The part for hook and function is blank in the manual…