The way the zombies generally work (i believe) is that multiple groups start at a town around the map.
They then behave as they normally do spreading and attacking towns.
The players forces then get a chance to act, but the time that has passed depends on the difficulty level.
1 day or normal, 1 day and 12 hours and hard and 2 days on nightmare.
As for why you can’t act at 0 or 6 hours, i guess you could and they would be very easy and easy mode respectively.
Given that you as the player are not playing as the units but as an all seeing overlord type you could see yourself in many ways.
For example as a divine being who finally answers the prayers of their subjects, or a powerful mage/king that finishes casting a spell to see the land and direct their troops.