Making a date object default to the last day of the month

hello all…question from a newbie trying to make a go at rails…

i have an account object which holds sundry information including a
credit card expiration date. i use date_select ( “object”, “method”,
:order => [:month :year] ) to allow the user to select the month and
year of his credit card’s expiration, but when this info is sent to the
model, somehow it defaults to a date object containing the correct
month, correct year, and the first day of the selected month. since
credit cards dont expire until the end of the month, i want to be able
to have the default action be to set it to the end day of the selected
month. i have figured out the logic if i have a date object…im just
not sure where i should go about putting the code that does this…any