The Imperial Calendar was instituded in the Old Republic year of 25,034. As he announced his new Empire, Palpatine also threw out the dating system the Galaxy had been content using for several thousands of years. At the close of the Clone Wars, it reverted to zero.
This calendar really only lasted barely passed year 25, as the New Republic swept away the last Imperial remnants after the Emperor's death in year 23. Rather than going back to the Old Republic dates, Mon Mothma did the unpopular move of forcing their own new calendar on the citizens, marking the destruction of the original Death Star at year 0.
The Imperial calendar outlived its Emperor by four years.