How to generate a date limit value in email confirmation?

For a registration form that sends automatically a gift certificate with limited use date, I am looking for a simple way to add to the confirmation email a field that would calculate automatically this date equivalent to the registration date +21 days.
Any tutorial for this?
Thanks for your help.

Hi faraon,

I found this StackOverFlow answer that you can use in a Custom Code action.

<?php
$date = strtotime("+21 day");
$form->data['expires'] = date('M d, Y', $date);
?>

Then use {expires} in the Email template.
Bob

Thanks a lot Bob, looks like the solution, but unfortunately I used "Custom Element (HTML/PHP)" and it does not work.
It does not reflect in the email any value (or even if I try the confirmation message)...

Hi faraon,

Please try putting it into a Custom Code action in the On Submit event of your form (or the Custom Code Before Email box if you are using the Easy Wizard).

Bob

Excellent, works great!
Thanks a lot.