User can submit post from frontend submission form. Fieldname:

Title (entered by user)
expire_date (entered by user)
expire_date1 (invisible)
expired (invisible)

When user enter no value for "expire_date" then i have to enter fallback value for custom field expire_date1. The fallback value is: current date + 90 days.

In next step: When date has been reached then change custom field "expired" from 0 to 1.

How to automate this writing and checking process?