The old Earth calendar was an Earth calendar used for dates during the 22nd century. For example, the years 2123 and 2190 were generally placed on this calendar. (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.