Thank you BlackMale for reporting this error, and Philipp for further explanation.
It has been fixed for UBB.threads 7.7.2
To avoid potential errors, the widely accepted date format of "YYYY-MM-DD" will be used. The $day and $month values coming from edit_user_sub.php will no longer depend on the language strings, and are now converted in to two digits going forward. (example: $day = sprintf('%02d', $day)) for passing to the strtotime() function which parses that format into the Unix timestamp.
