MacProject Pro uses calendars to determine the working schedule of a Timeline. ClarisImpact is not quite as sophisticated but limits can be specified so that the durations you enter are calculated on a 40-hour work week (skipping weekends).
1. Create a new Timeline Model
2. Click on Custom from the resulting dialog box
3. Change Days/Week to 5
4. Change Hours/Day to 8
While MacProject Pro can specify Holidays when determining durations, ClarisImpact cannot.