Hi, our team is also having trouble passing a trial start and end date via a form. Pretty much exactly what was described above - it shows in the form submission, but is not passing thru to the contact record, and they are receiving the following error:
This value wasn’t passed to the contact record. ‘5/7/2019’ isn’t a valid timestamp.
They’ve tried both with and without timestamp. Can you provide the format you used to solve this?