GstTestClock is an implementation of #GstClock which has different behaviour compared to #GstSystemClock. Time for #GstSystemClock advances according to the system time, while time for #GstTestClock changes only when gstcheck.test_clock.TestClock.setTime or gstcheck.test_clock.TestClock.advanceTime are called. #GstTestClock provides unit tests with the possibility to precisely advance the time in a deterministic manner, independent of the system time or any other external factors.