Prevent Existing Date from Being Overwritten by Default Date

I have an Edit page that has multiple stages to track a process. The intent is for users to mark a step complete as they progress through the process. They must also mark what date they completed the step. I've used Javascript to conditionally lock the steps so that they must be completed in order and I've set the default for each date to "Now()" so that by default it will be the day that they mark the step complete, whereby unlocking the the date field. I thought of hiding the date and having it completed automatically but since the users sometimes enter things a few days later, I need to give them flexibility to enter things in the past. For the most part, everything has been working fine but now I've discovered a problem.

The problem is that the default date of "Now()" is overwritting dates that were entered in the past. I would like to set an Event or something that would check to see if a date is already entered and, if so, leave the existing date, otherwise, offer today's date as the default. Is this possible?
Thank you in advance.

how about a series of edit pages you can re-direct to based on some value such as the date field being filled in
So user opens up original page but has already completed that section - during the loading of that page do a check for a value and re-direct to the next sequence of fields on a new edi page and so on - the if then else statement could than have a final if with an alert - job done or a redirect to a message page saying completed already etc