I'm not sure, but I think this has something to do with SMS that are stored on the SIM card.
I have a batch of text messages that are dated to the current date every time I do an OS upgrade. But at some point I had stopped savings messages on the SIM and those seem to be correctly dated.
I don't think downgrading the OS will put them back into their old dates. That info is still there since I can put my SIM into my Razr and they have the dates, but the BB seems to not see that data.