Buy the $500 Time Machines TM2000B. Turn on PTP multicast. Turn on Clock Synchronization in your PLCs (a checkbox in controller properties). For ControlLogix prior to L8x, enable clock sync in the network module connection in your I/O tree, too.
Sit back and watch all your Logix PLCs synchronize to within a few microseconds of each other.
The TM2000B is also an NTP server to which you can point your gateway (and other servers) to get millisecond-grade time synchronization.