Impact of adjusting Time of Mongodb server?
The impact of time adjustments depends on how significant the clock drift is between members of your replica set. Generally the most impactful outcome will be some confusing diagnostic information: operations during the affected period may appear to have improbably large (or small) durations.

If you are running any multiple server deployment you should always use a clock sync service (eg. NTP) to avoid potential clock drift. Generally services that make small, incremental adjustments to time (eg. ntpd) are preferable to utilities that make large adjustments (eg. ntpdate).

For more information on the potential outcomes of clock skew with MongoDB, see: MongoDB and Leap Seconds.