"The Julian calendar began in 45 BC (709 AUC) as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year (known at least since Hipparchus)."
"The Gregorian calendar reform contained two parts, a reform of the Julian calendar as used up to Pope Gregory's time, [...]. The reform was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio)."
Any rulers out there want a calendar named after them?