Green’s Dictionary of Slang

1. (Aus.) one month (in prison).

1848		Bell’s Life in Sydney 11 Nov. 3/2: She was knocked down as a lot exactly suitable for furnishing the Keck-ian establishment at Darlinghurst for two calendars.
1850		Bell’s Life in Sydney 20 July 2/6: The Bench ordered him to find sureties to ‘stick to his last’ for twelve calendars.

2. (US Und.) a year spent in prison.

1926		J.J. Finerty Criminalese 12: Calendar — Year and a day.
1931–4		D. Clemmer Prison Community (1940) 331/1: calendar, n. One year (sentence).
1966		M. Braly It’s Cold Out There 37: It’s rough [...] Rough as a cob. ’Specially for you — many calendars as you pulled.
1977		E. Bunker Animal Factory 109: I’ve been here eighteen calendars and I know how to get things done.
1989		J. Morton Lowspeak.
1999		W.D. Myers Monster 147: Some guys have done a whole calendar in here,’ I said. [...] I told her that doing a calendar meant spending a year in jail.
2009		W.D. Myers Dope Sick 31: You get caught with enough to deal and you catching calendars.

3. (US) a year spent in military service.

1988

W.D. Myers Fallen Angels 273: ‘Sucker get through a calendar, you ought to give it to him [...] You got to like a man make it through a whole calendar over here’.