Is there a function that allows the user to save an incomplete form submission and come back to it at a later date to finish it?

Yes, and we have two different solutions for you. I would recommend the first one since the data will be retrieve from the Jotform server, this also means users can use different browsers or machines to continue where they have stopped by using the session URL. The second method saves the data on the browser local storage which means they need to use the same browser and machine to retrieve the data.

1. http://www.jotform.com/features/save-and-continue/

2. http://www.jotform.com/help/227-How-to-Enable-Auto-Fill-Feature-Save-Form-Entries-to-Continue-Later

If you need further help, let us know.