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For: September, 2003

Use of Calendars in a Project Schedule
Submitted by: Thomas D. Fertitta

The use of a calendar in a schedule allows the scheduler to tailor the schedule to the specific periods of work during the project. A project calendar considers holidays, non-work periods, and specific days per week an activity is expected to be performed. Multiple calendars can be used to differentiate the work periods of specific activities or groups of activities that require separate coordination or contain different requirements and patterns.

A few examples of the use of multiple calendars to differentiate work include:

	weather related activities, e.g., no sitework performed during winter months,
	various activities requiring 7 days per week rather than 5 days per week in a fast-track project or for a procurement activity,
	international requirements requiring a different characterization of the work week, e.g., start the work week on Saturday rather than Monday, and,
	project-specific access requirements that limit work in specific areas to certain days of the week, e.g, the days a project area is available for work are Mondays, Wednesdays and Fridays.

Calendars play an important role in the accuracy of the schedule. They directly affect the calculation of the forward and backward pass in the determination of the critical path and overall project duration. An activity on a 7-day per week calendar with a duration of 21 work days yields the same forecast as an activity on a 5-day per week with a duration 15 work days. If the either activity is switched to the other calendar, the projected finish date would change, and if the activity was on the critical path, than the overall project duration would change.

Using calendars with different work-week durations can complicate the assessment of the project’s true critical path because differing work-weeks obscure the path with the least amount of float. Numerous calendars used in the same project, e.g., a 5 day per week, a 6 day per week and a 7 day per week calendar, can complicate float assessment even further. Scheduling software programs do not currently reconcile the float differences inherent in the use of multiple calendars. When using more than one project calendar to describe a work week, the true critical path is determined by comparing the start and finish dates of the activities across each of the project calendars and not the path with the most negative float.

An example of the complication can be found in the instance where a project is behind schedule and negative float is calculated. An activity sequence with minus 14 work days of float on a 7-day per week calendar has the same float as an activity sequence with minus 10 days of float on a 5-day per week calendar. At the same time, if an activity sequence on a 5-day per week calendar contains minus 12 work days of float, then those activities are the most critical even though they appear to contain relative float as measured against the activity sequence on the 7-day per week calendar with -14 work days of float. The determination of the most critical path becomes more complicated if a 6-day per week calendar is included.

Other complications in the assessment of the schedule may arise in the projections of resource forecasts and cost projections, which are often tied to an activity’s work day duration. 

In conclusion, the real world of scheduling may require the use of multiple calendars, however, care should be taken to minimize the number of calendars used and an extra step should be incorporated after each monthly update to accurately assess the project’s true critical path, resource forecasts, and cost projections.

About the Author: Thomas D. Fertitta is the Member-Manager of TDF, LLC, a construction consulting services company specializing in CPM scheduling. They are located in Herndon, Virginia.