The Julian Calendar, introduced B.C. 46. It fixed the ordinary year to 365 days, with an extra day every fourth year (leap year). This is called “The Old Style.”
The Gregorian Year.
A modification of the Julian Calendar, introduced in 1582 by Pope Gregory XIII., and adopted in Great Britain in 1752. This is called “the New Style.”
The Mohammedan Calendar,
used in Mohammedan countries, dates from July 16th, 622, the day of the Hegira. It consists of 12 lunar months (29 days, 12 hours, 44 minutes). A cycle is 30 years.
The Revolutionary Calendar
was the work of Fabre d'Eglantine and Mons. Romme.
A Newgate Calendar or “Malefactors' Bloody Register,” containing the biography, confessions, dying speeches, etc., of notorious criminals. Began in 1700.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894