Is the Navy 2 year time on station after a PCS applicable to ETS?

Original ETS should have been August 18, but my request for extension at my first command was denied and I was forced to accept PCS orders for NOV 2017-20. My understanding is that the Navy has a 2 year time on station rule and that my accepting new orders I am essentially involuntarily extended until NOV19. Is this true or legal?