When I made the ruling on Christianity and created the off-map concept, I assumed that Eras was the same size as Earth, meaning at the equator a time zone would cover 1669.79 km on average, and that the rotational speed of the planet would be identical to Earth. There should be 24 time zones since we're on a 24 hour clock, which means with our region's 13,500 linear km, we would have between 8 and 9 time zones covering our section of the world.
Now, if we decide that the off-map area isn't as big as the area of Earth not covered by the map, then the zones would have to be smaller, and the rotation speed of Eras would slow down as well if we wanted time to be similar to Earth.
As far as where to place the UTC zone, I would propose either the capital city of Imperium Augustum or the capital of McMasterdonia as the center of the UTC time zone. For the sake of simplicity, just draw the time zones as if the world was flat and just use straight lines. Don't adjust for distance to the poles, as the zones would be closer together as it approached the pole, in our case only serving to complicate the calculations.