Gregorian calendar

The Gregorian calendar was a system for measuring one solar year on earth. It was an improvement upon the Julian calendar. The Gregorian calendar was used on Earth from 15 October 1582 until the adoption of Stardates. (Excelsior: Forged in Fire)

In a minor capacity, the Gregorian calendar was still used to define years as late as 2379. (TTN novel: Taking Wing)