A "day" is still 8 hours. If all of your tasks are based upon a 10 hour work day, you should change the definition of "day" in Tools > Options. Schedule. However, if you have a mixture of calendars, some with 8 and some with 10 hours per day you're going to have to choose one definition.

I suggest stopping talking "duration" and start talking start and end dates if you have mixed calendars.

Julie