Time zone of the server gets reverted if i set timezone in plesk

[balechaparka]

From about last year my server time zone was set to utc+5:30. I had done this changes in plesk "set time" option. It was working fine for last one year. Yesterday after restarting my server i saw that my server time zone is changed to utc-7 . So i changed the time zone in plesk to utc+5:30. When i see in server the time zone changed to utc+5:30 but after a minute automatically it get reverted to utc-7.So now if change time zone in plesk then it will effect server only for about a minute and then time zone gets reverted. Then i changed the time zone in server itself. But same problem gets repeated. So now i am not able to set time zone to utc+5:30. I tried it several times. I have another server with same configuration and settings. That server also i restarted and checked time zone and is working fine. but only this server has this problem.So why time zone does not change if i set time zone in plesk? So please help me to set the time zone properly.
Thank you