custom hidden field

I am using a form to generate a coupon for site visitors. The coupon should “expire” one month after they fill out the form…so I need a way in the email that they get, to list a date that is one month in the future. I assume that placing a hidden field within the form and setting the future date on that field is the way to go, but I am not making much headway. I have a hidden field called hidden-expdate

I have tried setting up some javascript (not my strong suit) with on_submit in the Additional Settings tab…but no luck thus far. Can someone point me either in the way of making this approach work, or suggest an alternate approach?

on_submit: “
var now = new Date();
expdate = new Date(now.getFullYear(), now.getMonth()+1, now.getDate());
this.form.elements[“hidden-expdate”].value = expdate;
return true;
“

https://wordpress.org/plugins/contact-form-7/