Historical clue formulations

• 21-Across's "Three Pieces in the Shape of ___" — 2016-07-22
• "... in ___ tree" — 2009-07-05
• "… partridge in ___ tree" — 2006-07-31
• "...in ___ tree" — 1998-11-22
• "...in ___ tree" — 1997-12-25
• "...partridge in ___ tree" — 1997-02-20
• "... partridge in _____ tree" — 1994-06-08