Old Calender was a term used in the 23rd century to describe the Gregorian calendar system that was used on Earth for centuries. The Old Calendar was commissioned by the human religious leader Pope Gregory XIII in 1582, as a reform of the older Julian calendar. The calendar was originally meant to measure the number of years since the traditional year of the birth of the human religious figure Jesus Christ.
The Old Calendar system was later partially superseded by the stardate system, as this calendar system proved too impractical as humans spread out across the Alpha Quadrant and lived on different planets with different rotations. However the old calendar was still widely used by humans. (TOS novel: The Entropy Effect).