657,209 AD is the year visited by The Time Traveler on one of his visits to the future.
During the Time Traveler's first trip into the future, he passed through this era without stopping, but he certainly didn't notice the sun going out; This caused much surprise upon his arrival during his later trip to this year. ("The Time Machine")
After telling his friends in the 19th century about his trip to the future world, he unknowingly created an alternate timeline. In this new timeline, humans had constructed a massive sphere around the sun, which became their new primary habitat. Physically, they had genetically engineered themselves into Morlocks by coincidence; they had never encountered the Morlocks from the original timeline. Nebogipfel befriended the Time Traveler after he arrived in this year and became his time traveling companion. ("The Time Ships")