Time moved at a relative speed depending on where one was. No one understood it very well, but it was different between worlds, ever so slightly such that one couldn't notice it on a day-by-day basis.
If a single day passed in one world, in another world, it may have been a day and a few minutes. Then, in another world, it may end up being a few minutes less than a day.
There was no crazy difference, but enough so that within a single year, the words would be apart by a day or two.
For mortals who never left their world, it did not matter. For Immortals, it still did not matter as they never cared much about the passage of days. They marked their time in months, if not years, so losing a few days was nothing to them.
However, it did matter when it had to do with things that were counted in days, like the Interrealm Teleportation formation.
