Historical clue formulations

• Zero — 2018-05-05
• Emphatically zero — 2016-11-01
• Zip — 2011-08-13
• Zero — 2009-12-08
• Opposite of all — 2004-03-21
• Nil — 2003-12-25
• Zero — 2003-05-13
• All's opposite — 2002-03-17
• Opposite of all — 2000-02-01
• Zero — 1999-01-20