Migrating Data From Old Version to New

Hey guys,

I placed this in the PHP section because I built this site in PHP, but my issue has mainly to do with data migration.

I'm planning on doing a gradual migration of all users from the old system to the new on http://www.sitdiary.net (basically, a custom-built community of blogs).

At the moment, I'm still building the new site but I'd like to finish the migration tool.

My problem is that when I built the old version, I was ignorant and didn't build a proper date field until much later. The old version allowed the users to change the date to be anything they wanted. So now the possibility has come up of having hundreds, perhaps thousands of posts that lack a date field being transported to a new table that requires a date field.

When users want to migrate their old posts to the new database, they're going to be upset if they have to manually set the dates to all of their old posts. I'd like to avoid this scenario, if possible.

Has anyone run into this problem before? I'm trying to come up with a creative solution that will be easy on my users. After all, I'd like them to all migrate without a hitch.

Thanks in advance!