Not hard, visualize the locations on the globe and a pie with 24 slices. If you start work at 12, and you want to know when someone 2 slices West will start you add 2 to get 14. 2 slice East of you, subtract 2 to get 10.
Better than guessing what timezone the region picked when it spans multiple natural time zones, and whether they do or don't have time changes.