If the MX time is fast, the NTP server won't correct it automatically because by backing up the time will cause the MX delete records during the time frame being deleted. This has to be corrected manually. You can then add an NTP server to keep the time in synch.
1. remove the NTP server
2. save the change and reboot the MX
3. add the new NTP: time-a.nist.gov
4. save and reboot
The time is now correct and will stay correct.