The top two sets of options on the Calendar tab set the day on which weeks start, the month on which years start, and the default starting and ending times for tasks . The lower portion of this dialog can be a bit confusing. One way to lessen this confusion is to think of this last section as dealing more with conversions rather than calendars because it's really defining how Project will convert work or duration entries you make.
For example, the default settings are 8 hours per day, 40 hours per week, and 20 days per month. This means that if you enter a duration of one week, Project will "know" that you are really saying 40 hours. Likewise, if you enter a duration of one month, Project will know that this means 20 days, and that those days are 8 hours long.
Set the Calendar Options
Use these controls to specify on which day the week starts and on which month the fiscal year starts.
These controls are used to define the default start and finish times for tasks entered without specific times. For example, if you simply enter a start or finish date but do not enter a time, Project will use these times.
This setting controls how Project converts the concept of a day into hours. Project assumes that a day is eight hours long by default.
Use this setting to change how Project converts the concept of a week into hours. Project converts a week into 40 hours by default.
Project converts the concept of a month into 20 days by default.
Click Set As Default if you want to use the current settings as the default for new projects.