MMLI

Set to 2 different clues across 2 published puzzles, most often “Mid 21st-century date” — The New York Times, Tuesday, 27 March 2007.

4 letters

Every clue on record, most used first

Mid 21st-century dateonce
The New York Times — Tuesday, 27 March 2007
Year in the middle of this centuryonce
The New York Times — Sunday, 11 March 2007