Facebook Timestamp Question?

[Junie]

So the problem occurred yesterday when I was messaging a business partner.
On facebook, when one uses the mobile, there is usually a sign next to their name that states their last log-in. The time I messaged her, it showed she logged on 4h ago. On the next day when I woke up, I went to check to see whether she got the message yet. To my surprise, this time it showed 2d meaning she logged on 2 days ago! Is this facebook's technical problem? It's only been 8 hours since I last sent the message, how did 4h become 2d?

Thanks!