If your clocks could be controlled by computer somehow, NTP (Network Time Protocol) provides a traceable standard time reference.
If you're feeling ambitious, and your clocks are on their own feed, here's a pointer to some accurate 50Hz inverter solutions: https://bestengineeringprojects.com/crystal-controlled-inverter-verified-inverter-circuit/
Combine this with an NTP client to make the 50Hz to close the loop.
Maybe a UPS could do this as well, provided it uses an accurate time reference. This solves a secondary problem of power loss.