We had an interesting anomaly with GLUT/Intel/Linux this week.
After setting the CPU time back 2 minutes, our GLUT window would not update for 2 minutes.
Does GLUT use the CPU time for updating the display?
The freeglut git repo shows added support for CLOCK_MONOTONIC in 2012.
If you have a version of GLUT that uses gettimeofday instead of clock_gettime(CLOCK_MONOTONIC), code that is driven by fghCheckTimers would probably behave like you describe when setting the time back.