Historical clue formulations

• Geological period suffix — 2016-06-14
• Suffix on era names — 2010-01-31
• Suffix with Meso- or Paleo- — 2008-04-30
• Suffix on era names — 2004-01-18
• Suffix in the names of many eras — 2002-12-14
• Suffix on era names — 1999-10-20