In dating systems, BCM, as compared to ACM, refers to years “Before Christ Myth”, according to the Paine dating system (1802); which is a secular alternative to the Roman recension myth-based BC (Before Christ) or BCE (Before Common Era) Exiguus dating system (525AD) notations.

Further reading
● Thims, Libb. (2016). Smart Atheism: For Kids (pdf | 309-pgs). Publisher.