changing date in server automatically

Hi, Tried to put some m/c in date&time synch, which are in one N/W. Followed these steps :- 1) open date & time settings. 2) set same date & time as well as same time zone. After 3 days it is found that, one of the m/c date has changed. it is showing yesterday date. Is it possible that, the date can be changed automatically! Am sure, no one logged in and modified that. Thanks in advance.

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