When is a playlist is scheduled to play at certain times, it is always started at 0th second of the minute. Using this feature, you can synchronise playlists across players.
Schedule playlists under all Groups to start at a specific time everyday (say 8AM). Make sure all the players are up and running before that time (8AM). At 8AM (in this example) all the playlists will get scheduled and start at 0th second and hence will be in sync.
If you want to deploy new content please do so before the start time (8AM in this example). After deploy, playlists will not be in sync till the next one is scheduled.
Also, players need to be in sync with NTP server time.