So I'm not Moose, but I DO have the Encyclopedias!

In the first one there are two different versions of Midgarsomr's appearance on Hydaelyn. According to the Twelve creation myth when water was created by the gods Midgarsomr rose from that, which would put him in the First Umbral Era after the split. HOWEVER, they also have an account from dragons that he came from "a time before time" when Hydaelyn was still a newborn star. Which for me would put him as having come before Hydaelyn sealed Zodiark as a moon.