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In the thread about dating systems, somene suggested a possible "Year Zero" could be the year Temujin united the Mongols.
I suggested that would only work if the Mongols had gone on to establish a polity that (eventually) unites the entire human species.
So...
Your challenge is to make it so the Mongol Empire eventually unifies the human race. The splintering of the empire into separate khanates would make this a problem, but if there was some kind of "Mongol Commonwealth" or confederation in which the khanates cooperated with each other and only expanded outward, that might be doable.