Historical clue formulations

• "___ but known ..." — 2012-01-23
• "___ known then ..." — 2007-09-23
• "___ known ..." — 2001-09-16
• "___ known then..." — 2000-06-18
• "___ but known..." — 2000-05-21
• "_____ known then what..." — 1994-06-20