Restore Original 'join date' for members

[Electronic Punk]

Earlier this year my site suffered a serious database problem and we had no real backup solution in effect (oops) and we made the decision to start the forums from scratch with vbulletin3 in order to get the site up again as fast as possible - a very painful decision.

We basically lost all vb2 tables past subscription, alphabetically, so we started from scratch and things started getting back to normal with old users signing backup.

We also started work attempting to get the vbulletin2 database functional again and eventually we got it to such a stage where we could repair the database so that the forums could be viewed.

Unfortunatly the users table was also lost so alot of information such as email address, password could not be regenerated.

It did mean we were able to restore the old posts back into the new vbulletin3 forum using the Jelsoft import tool and effetively all the old users post counts were restored.

All the old posts were left into one forum (we had to regenerate the threads table) but we didn't know which forum id the threads were in so they were all moved back into one big forum composed of around 35,000 threads.

This was a few months ago and now only 7000 threads now need to be moved before all the old data is back in the main forums (sadly old users are guests, but I guess we have to live with that and I can restore them if a new user lets us know that he was here before and I can work around it)

What I would like to be able to do, to get a users join date back as close as possible to when they joined is to base it from their first post.

I am not too hot as SQL so don't really know how to do this, but alot of user think it is important.

I can only think of a few things like if the user has never posted on the new forums then their join date shouldn't be adjusted. But some join dates could be adjusted by over 2 years and it would be great if there was a way of doing it.
Cheers.