tp="" short code for free trial in Pro Forms

One thing you could try is setting tt to “D” and then tp to the number days until the date you want.

I don’t remember if PayPal lets you go over the 7 or 30 days trial before it requires you to use weeks or months instead of days as the unit…

Enable logging and test with a 1 cent transaction if it works, for example, with a 100 days trial, and you don’t get any errors in the log. WP Admin -> s2Member -> Log File

If it works, then you’d need some PHP in your shortcode to calculate dynamically the number of days before the date you want.

These may be useful:
https://wordpress.org/plugins/ezphp/
http://php.net/manual/en/function.strtotime.php
http://php.net/manual/en/function.date.php

I hope that helps. 🙂

Cristián