I'm pretty sure there is a post about this but can't find it.
I think the reason is that when the "old forum" posts were copied across, they only had a date, and thus the time was set to zero. So rather than suggest that everyone posted at midnight, the time is suppressed if zero.
I remembered one too, but I couldn't find it either. And now that you explained it to me I remember the reason also. Hmm, could it have been me who reported it before? Fishy..,
Thanks!
[EDIT]: This was my 2011th post (as in the current year). Hmm, I was hoping it would be a little more useful.
As a somewhat hacky solution, perhaps when posts are created you could check if the timestamp is exactly 12:00 AM. If it is, bump the minute up one. It only affects new posts and leaves the legacy ones alone.