Historical clue formulations

• Bygone West Coast conference name — 2023-04-05
• Previous name for an athletic conference now with 12 members — 2018-05-13
• Conf. whose membership increased by two in 2011 — 2014-04-04
• Wildcats and Cougars play in it — 2009-03-13
• Washington and Oregon are in it — 2007-08-09