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year zero

noun
  1. the beginning (1975) of the period during which Cambodia was under the control of the Khmer Rouge
  2. the beginning of revolutionary change
  3. the beginning of any new system or regime

Word Origin for year zero

C20: by analogy with Year One of the French Revolutionary calendar
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012