Historical clue formulations

• Quadrennial occurrence — 2024-04-21
• Quadrennial bonus — 2021-09-24
• February 29 — 2021-02-08
• 2/29/16, e.g. ... or a hint to the circled squares in this puzzle — 2016-02-29
• March preceder, periodically — 2009-03-13
• Bissextile occurrence — 2002-06-30
• February 29 — 2002-01-07