Historical clue formulations

• Either 1 in "1+1" — 2020-03-04
• Part of the total — 2013-03-29
• Number next to a + — 2011-07-27
• Part of a sum — 2006-07-17
• Number on either side of a + — 2006-01-17
• The 3 or 5 in 3 + 5 = 8 — 2005-11-21
• Part of a column — 2004-07-17
• Number next to a plus sign — 2001-04-05
• Number next to a plus sign — 1998-11-01
• Arithmetic figure — 1994-08-12