As the Clone Wars ended, the calender restarted at 0 according to the Imperial Calendar. Mon Mothma decided to act on the unpopular concept of starting yet another reboot with the New Republic Calendar, with it's zero year happening at the destruction of the first Death Star. This was Imperial year 19 (or 25,053 in the Old Republic Calendar).
Once the Emperor was killed, and the final significant remnants of the Empire was swept away (I'm not talking Thrawn or any of those fools, just the ones who couldn't let it go and kept at it like a retarded person in bed who didn't notice your ugly O face five minutes ago), the Galaxy began the long slow process of adopting this New Republic Calendar. It lasted until some other jerk decided to restart with some other somewhat notable event.