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")

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.
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