Synchronize Arduino times to a 20ms difference
"NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve 1 millisecond accuracy in local area networks under ideal conditions"

For example: Arduino UdpNtpClient - though this isn't a full NTP client and doesn't maintain accurate time continuously (unlike a normal NTP client)

Network latency isn't a problem, jitter may be. NTP requires the systems be able to communicate at regular intervals and requires time to settle on accurate values.