Time cannot go backwards - but sometimes, they do in logs.
Most of the times, it happens when use a log from an Apple. The timer there is so unreliable that they synchronize hourly with time servers, and every time that happens it always jumps either forward (=ignored) or backward (=problem).
If you encounter warnings on the site saying that the time was adjusted to fix some problems, don't be too sure the time is 100% correct; and if it jumps during a fight, it could be broken apart into two.
If this happens frequently with your logs, either disable time synchronization or get someone else to log - your computer is doing a lousy job of tracking time. I've seen logs where time is set backwards for 3 minutes every hour - even the cheapest mechanical watch is more accurate...