The sutta ties Metteyya's arrival to a moral and demographic condition, an eighty thousand year life span, not to a calendar date, and offers no way to compute when that condition will be reached. Popular Chinese movements have repeatedly read it as imminent regardless. From around the thirteenth century, White Lotus teachers preached the near arrival of Maitreya, at times fused with Manichaean imagery of a coming Bright King, and organised that expectation into a standing underground network active for centuries.
The clearest case of the belief meeting political history is the mid-fourteenth century Red Turban Rebellion against Mongol Yuan rule, which drew heavily on White Lotus Maitreyan preaching. One of its commanders, the former monk Zhu Yuanzhang, went on to overthrow the Yuan and found the Ming dynasty in 1368. A later revival, the White Lotus Rebellion of 1796 to 1804 against the Qing, was suppressed after eight years of fighting. In neither case did the specific condition the sutta describes, an age of eighty thousand year life spans, arrive, and the movements' own leaders did not claim it had; what each generation read as urgent was the idea that the age was close, not that its stated precondition had been met.