Occasionally, when I close my laptop and revisit it later, the clock on my system tray will be a few hours behind.

I have the computer set so that the laptop does NOT sleep/standby upon being shut. Instead, the monitor is just turned off.

Any information on what may be happening?

You need a new clock battery. There is a little battery on your motherboard that powers your clock, go to your local PC shop and ask them for one.

I had no idea that the clock even ran on a battery. For a while I was thinking absurd things like someone had hacked my computer, and my clock was coinciding with their time.

Thanks for this .

No problem.