There is a capacitor in the system board that keeps the clock running even after the power is turned off. If the PacketShaper is not powered back on before the capacitor is out of power, the time in the PacketShaper will revert to May 14 1987.
If you see the above problem, you should reset the date and time. If you keep your PacketShaper powered on, everything should work fine. Normally, the clock should run fine for several days even without power.