Problem:
Post expiration date in form doesn't work anymore -set post expiration time using generic custom date field

Solution:
Toolset forms saves the post expiration time to postmeta table with the key "_cred_post_expiration_time". To override the value of post expiration time based on the custom date field value, you will require to use the Toolset form's hook "cred_submit_complete".

https://toolset.com/forums/topic/post-expiration-date-in-form-doesnt-work-anymore/#post-2072441

Relevant Documentation:
=> https://toolset.com/documentation/programmer-reference/cred-api/#cred_submit_completecre