Historical clue formulations

• Guarantee — 2018-09-16
• Solemnly affirm — 2014-02-17
• Avow — 2010-09-03
• Certify — 2007-07-20
• Affirm — 2004-12-03
• Solemnly affirm — 2003-02-16
• Affirm under oath — 2001-09-17
• Solemnly affirm — 1999-05-10
• Affirm under oath — 1997-12-21