Adding value to date to get a date

[Grendor]

For example if I want to input activation date(or credit that determines date of expiration) of my prepaid cell phone card, and based on that and current date php script calculates what is the expiration date.
example:
50$ equals 100 days of my prepaid cell card,and based on that and the current date, what is the date when my prepaid card expires?

Any ideas are welcome.
Thanks.

[lilleman]

$expires = date('Y-m-d', strtotime('+100 days'));