Are you working with Project Online/Server or just the standalone desktop version?
Calendar instructions are here Changing working times
If you're using Project Online/Server then the Enterprise calendars probably need to be updated for the benefit
for all.
You will also find that if you're moving away from the default 8hrs day 40 hr week you will need to update the working time settings of project professional itself via File>Options>Schedule. If these don't match then you can experience
some interesting calculations around resource costs and work etc.
Hope this helps
Matt
