Some external software processes is changing the time of system.after deletion of some external packages now time is stable.

Thank you all for the support.

If no process is running to change the system clock which is typically ntpdate or ntpd then the only time it is set is at boot up. I don't see a system cron job. If cron and ntpd are not running then there could be some other process.

Have you examined the logs to see what changed when the time changed? At the moment the only thing I can suggest is stopping a process one at a time, change time and see what happens.

I see that you found the process.