Edit Entry (Once Only)
I have a form where the user enters an opening date in one field and a closing date in another field.
At the time they submit the form, they don't know the closing date.
Later on when they do know the closing date, I'd like the user to be able to go back into the system and Edit their entry, enter the closing date and upon updating the Entry, lock the form (or specifically just the Closing Date field) from any further editing.
I think you should do this:
1. Make the form available to only logged in users, so everyone has to create an account. Choose this via the Form Settings options.
2. Add Edit Entry link to the bottom of the form via the View settings of the form, preferably the Single Entry view. You also have to select the option to 'allow users to edit entry' when you click on the gear-icon of the Edit Entry tab in the Single Entry view.
After you've done these, any form user can login, search for the entry and edit whenever.