YM is the standard abbreviation following year notations in Six Species calendars, for example, "3865 YM".

Said to be adopted from the Molran "Firstmade" calendar but also coinciding with the human 12-month Gregorian calendar used in most regions of Earth until its destruction, "YM" stands for Yeka Mogak, a Xidh phrase meaning literally the Six Species.

The YM count therefore refers to the number of years since the official formation and drafting of the Six Species charter and the adoption of the human race as the sixth species.