Exactly.

You don't need a GPS atomic clock, you can actually buy a rackmount one.

If that doesn't work you can use the internet to connect to NIST.

NIST is actually where GPS get's it's time, so you can just connect, using the internet, there directly.

There are systems that can do a fallback to a digital radio timecode. These are extreme cases that DO NOT apply to just an ATM cash draw.