I've noticed this to be a frequent problem, and now that it's becoming a bigger problem as I use it to keep tabs on my staff, I thought I'd report it
Every once in a while (how often I really couldn't say) it sets all of my offline contacts that haven't been online recently to a specific time. So for example it will just say that they all were last online 6 hours ago.
Of course this isn't true, so it must be a bug
thought it best to let someone know.