Historical clue formulations

• Often-replaced reference works — 2016-08-27
• Yearbooks? — 2016-08-18
• Targets of fact-finding missions? — 2011-05-14
• Output from Benjamin Franklin's press — 2008-11-13
• Data recorders — 2007-04-06
• Popular references — 2002-01-13
• Astronomical data providers — 1998-12-31