[kitteh]

They could use their own timing source, though? I recall back in the day being able to pull timing source off a circuit.

[timdux]

Yes, that's the clock holdover circuit, usually built using a OCXO or double OCXO or rubidium clock. It's good for 4 to 24 hours. Example:

[0] https://www.rakon.com/19-rak-product/rak-product-ocxo