There are three different calendars in MS Project
- Project calendar - defines default work days and hours
- Task calendar - used when the timing of a task must be different than the project calendar allows
- Resource calendar - defines when resource is available
MS Project has rules how the calendars interact:
- if a task has no resources and no task calendar, it will use the working days and times in the project calendar
- if a task has a resource, it will use the working days and times fro the resource calendar for that task's resource(s)
- if a task has a task calendar, it will use the working days and times from that task calendar
- if a task has a resource AND a task calendar, it will use working days and hours the task and resource calendars have in common (unless the Ignore resource calendars option has been set for the task)
If the working day is adjusted on the standard calendar, be sure to adjust how many hours is in a day in the calculation MS Project uses.