8
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Excel, 1243 1206 1180 bytes
-37 bytes by subtracting from all the dates
-26 bytes by by fiddling with the subtraction values
=CHOOSE(MATCH(A1-65^4,{39805,119679,159679,239679,319679,399679,439679,519679,559679,559779,599679,639679,650084,679679,719679,759679,799790,839679,919679,959679,960294,9...