You are correct. You can only set one defintion of "hours per day", "hours per week", and "days per month." So when you enter a task duration of 10 days, Project translates that to 80 working hours (assuming 8 hours per day). It will schedule the task based upon the working time in the calendar that is controlling that task.

The attached picture shows you the impact of the task calendar with a 12 hour per day, 6 days per week.

If you are going to have many circumstances where you are going to have different calendars (different hours per day) - you need to likely stop talking duration. Talk start dates, finish dates and work.

Attached Images

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