Historical clue formulations

• Nine-sided figure — 2025-12-22
• Shape of every Baha'i temple — 2018-05-27
• A regular one has internal angles of 140 degrees — 2000-01-30
• Odd-shaped figure — 1999-09-15
• Nine-sider — 1995-06-08
• Defective stop sign? — 1993-12-16