They're a Millerite splinter not Mormon when the Millerites imploded after multiple failed predictions it created several new religions, the biggest two being being Jehovah's witnesses and Seventh day advenentists. They came out of the same religious spring as the Mormons and Christian scientists for example but are more or less unrelated outside of being nominally Christian. There used to be more groups from that era but most of them were Christian socialists and were more or less all killed off by the 1950s.

Sorry about the pedantry but my kin have been fueding with the fucken Seventh Day Adventists since they moved into what is now Loma Linda California back in the early 1900s.

There was big end-of-the-world hysteria when the year 1000 was coming up. And dozens of dates after that.