The Network Time Protocol (NTP) is used to provide accurate and synchronized time across the Internet.
NTP is a fault-tolerant protocol that automatically selects the best of several available time sources to synchronize to. Multiple time sources can be combined to minimize the accumulated error.
NTP has a feature to prevent time adjustments from time sources that begin sending incorrect time. If an NTP client detects that an NTP time server it is configured to get time from is more than 1024 seconds (or about 17 minutes) off from the NTP client’s time, then it marks the time source as insane (as opposed to sane) and will ignore that time source.
That means that when you configure and start an NTP client, you must ensure that the time on the client is with 1024 seconds of the time on the NTP time servers otherwise all the NTP time servers will be marked insane.