Historical clue formulations

• Prime meridian std. — 2021-05-20
• Prime meridian std. — 2020-09-26
• Prime meridian std. — 2020-07-26
• Astronomers' std. — 2020-04-15
• Astronomers' std. — 2016-04-23
• Astronomer's std. — 2015-05-10
• Prime meridian std. — 2014-10-28
• Setting at 0 degrees long. — 2013-09-15
• Prime meridian std. — 2013-09-06
• Prime meridian std. — 2013-07-04
• Astronomer's calculation: Abbr. — 2012-12-01
• Astronomical observation std. — 2012-05-22
• Prime meridian std. — 2011-11-09
• Prime meridian std. — 2010-01-06
• Prime meridian std. — 2009-08-09
• Prime meridian std. — 2009-07-05
• Setting at 0 degrees long. — 2007-03-03
• Prime meridian std. — 2006-10-15
• Standard setting at 0° long. — 2006-07-14
• Prime meridian std. — 2006-06-12
• Clock std. — 2004-02-01
• Standard setting at 0 degrees long. — 2000-04-07
• Clock std. — 1998-05-24
• Astron. clock setting — 1996-09-15