Old Earth calendar
The old Earth calendar was an Earth calendar used for dates prior to the 24th century.
In 2268, James T. Kirk requested that Doctor Leonard McCoy be outfitted like a Gestapo doctor from Nazi Germany with the rank of colonel, circa old Earth date 1944. This was done so that he could infiltrate Ekos. (TOS: "Patterns of Force")
USS Enterprise-D's computer placed the years 2123 and 2190 on this calendar as well. (TNG: "Up The Long Ladder")
In an alternate timeline, Captain Harry Kim on the USS Rhode Island had a Master of Space Science degree from Starfleet Academy, that was granted on stardate 48416.0, and old Earth calendar date of June, 2nd 2371. (VOY: "Endgame")
The old Earth calendar was initially referred to in the script of TOS: "The Galileo Seven", with only one reference to it in that teleplay. For the final version of that episode, though, that reference instead became a reference to a calendar system called the Old World calendar.