Yes, you can just edit the engine/settings.php of 1.9 to point to an existing database.

If location of the datadir or the site URL are not the same, you must update them as instructed here: http://docs.elgg.org/wiki/Duplicate_Installation#Database_Entries

Ok great, thanks for the quick reply. I guess I'll give it a try then.

Remember to keep a backup of everything in case something goes wrong during the upgrade.

The strange thing is that removing and uploading a new avatar image results in the exact same issue.

Thanks for the help though, I'll try and see if the options mentioned in that link help at all.

This affecting only a single user would indeed point to a timezone issue. That user has probably been created somewhere around 12:00 AM (midnight), so the timezone change causes the day to change completely.

Have you recently moved your site from a server to another? Or made any changes that might have affected the server timezone?

That's the strange thing, I never changed anything or switched timezones. It just started from one day to the other. However since the site runs on a shared host, the hosting company might have changed something...