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Friday, March 27, 2015

Duration Demystification

When I was a new user of Project, the term “duration” was a very strange notion to me. How can a task be 5 days in duration when starting on a Thursday and ending on the following Wednesday? That is a 7 day time period! It just didn’t add up.

The answer is perspective. From the perspective of work, the 7 day time period contains 5 work days and 2 non-working days. From the calendar perspective the same 7 day period is measured in consecutive days from the start to the finish. Both are legitimate expressions of duration but used for different purposes. Working durations are for team scheduling. Elapsed durations are for other scheduling purposes. This blog entry illustrates both.

A few definitions are in order. Project knows to use elapsed durations when the letter “e” is entered prior to the unit of measure. The following table should help you understand it.

Duration Entry:


1 Day (“1d”)

1 workday of 8 hours

1 Elapsed Day (“1ed”)

1 calendar day of 24 consecutive hours

1 Week (“1w”)

5 workdays of 8 hours each

1 Elapsed Week (“1ew”)

7 consecutive calendar days of 24 hour periods

1 Month (“1mon”)

22 workdays of 8 hours each

1 Elapsed Month (1”emon”)

30 consecutive calendar days of 24 hour periods.

Durations are expressed in minutes, hours, days, weeks and months. Each work duration has a commensurate elapsed duration. Note however that the units of measure in the “Meaning” column above are different, and need to be considered when assigning resources.

For example:  A resource assigned to a one day (“1d”) task will be assigned 8 hours of work. If assigned to a one elapsed day (“1ed”) task, the resource will be assigned 24 consecutive hours of work.

When scheduling tasks confusing work duration and elapsed duration can have an adverse affect on the length of a project. In the figures below I have included both types of durations as delay and included the delay as a task in the dependency chain. Note that the delay expressed as a normal duration is longer than the delay expressed as elapsed duration and thus two different project durations are possible. (Click on the images to enlarge)



Now that you’ve seen the two duration types used by Project mindful consideration should be given to enter the appropriate duration value in the task duration field.

Good luck on your projects! If you found this blog entry informative, send me an email, won’t you? I love to hear from my readers.