Historical clue formulations

• Like chalk on a chalkboard — 2019-06-29
• Unlocked, as a computer file — 2013-08-08
• Like some compact discs — 2012-11-07
• Not permanent — 2011-08-28
• Impermanent — 2010-06-11
• Like some pens — 2008-04-27
• Not permanent — 1998-04-24
• Penciled-in — 1994-12-30