Crab Nebula Time Period I've learned that the time period for a rotating neutron star, i.e., a pulsar is very accurate and can be used as an interstellar clock. But according to Walter Lewin's 8.01 Lec 19 on classical mechanics, he says that the rotational time period of neutron star in the Crab Nebula loses up to 364 ns everyday in the form of neutron star emissions (this comes from rotational kinetic energy of the neutron star). So, how are pulsars used as precise clocks?
All pulsars are observed to slow down gradually,albeit with a wide range of rates.The following diagram,known as the $\dot{P}-P$ diagram for pulsars illustrates this.The crab pulsar yes indeed would be a bad choice as a clock due to it's high $\dot{P}$.

But there are pulsars to the bottom left of the diagram which have periods of the order of milliseconds(hence called millisecond) pulsars and their $\dot{P}$ is also very low.These are the pulsars which can be used as atomic clocks,for example:MSP J0437− 4715.






  We can see that the both the period and the period derivative of this
  particular pulsar are very low providing great precision to the period
  of the system.So it can be used as a precise clock.


References:

1.http://www.cv.nrao.edu/course/astr534/PulsarTiming.html

2.http://www.cv.nrao.edu/course/astr534/Pulsars.html

3.http://en.wikipedia.org/wiki/PSR_J0437-4715