How is the Timer class in Java sensitive to the system clock? [duplicate]

Answer 1

Looking at the source, The Timer class schedules tasks using System.currentTimeMillis(). The ScheduledThreadPoolExecutor uses System.nanoTime().

currentTimeMillis() will tend to use the OS clock, the one that tracks the current date/time. nanoTime() will tend to use a higher resolution hardware clock.

If you move your OS clock back an hour, then currentTimeMillis() could should reflect that while nanoTime() should not.