The clock drift in FusionReactor is a check performed once per minute, we take the last recorded time and then again check the system time and compared the two values. If the clocks are not within a margin of error of the correct time then a clock drift warning will be generated.
Clock drift on your server can be caused by many factors, typically these factors are:
Virtualization technology synchronizing the machine time periodically and causing the clock to jump
A system under high CPU load that is unable to increment the time, when the CPU is freed time then jumps
An issue with the timekeeping daemon on your server
Unfortunately, there is no straightforward fix for this, typically using an NTP daemon is the best way to stop clock drift from occurring.
You can find more information in the documentation.