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Duration calculation when using 2 different Calendars in MS Project

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Éamonn O'Standúin
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Joined: 1 Feb 2019
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I'm hoping somone can help me with the following problem I'm having with MS Project. I have a project with Construction Activities (Standard Calendar appropriate) and Shutdown Activities (operating over 24hrs, 7 days a week calendar).

I can create calendars and apply them to the correct activities but the Duration field in MS Project calculates the Durations (in my opinion) incorrectly? I can't seem to get my desired result to display of 1 calendar day.

The Duration will calculate as 1 day if I use the Standard Calendar but will calculate my Shutdown activities (using the 24hr calendar) as 3 days even though this is happening from midnight to midnight. It is effectively calculating 3 shifts of work rather than Durations of days. If I change my working day in 'File - Options' to 24 hrs, I will then get that it calculates the Construction Activities (Standard Calendar) as 0.3 days and my Shutdown Activities as 1 day. See below of the undesired result.

Construction Activity A

Start 01/02/2019 8am  Finish 01/02/2019 5pm  Duration = 1 day

Shutdown Activity B

Start 01/02/2019 00:00am  Finish 01/02/2019 23:59pm  Duration = 3 days


Ideally, I want to see my Durations as 1 day for both tasks as they are happening over the course of 1 calendar day so I have:

Construction Activity A 

Start 01/02/2019 8am  Finish 01/02/2019 5pm  Duration = 1 day

Shutdown Activity B

Start 01/02/2019 00:00am - Finish 01/02/2019 23:59pm  Duration = 1 day


Any help/guidance on this would be much appreciated.





Tom Boyle
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Joined: 28 Nov 2006
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You are describing a natural consequence of using multiple calendars in MSP, where...

1. Any "duration" field refers to working time, not calendar time.

2. All durations are stored in minutes.  They are converted to days (or weeks/months) using the Project-specific conversion factors found at the top of the Schedule tab in Project Options.  (This is unlike P6, which has calendar-specific conversion factors.)

3. As a semi-exception, MSP also includes "elapsed" duration types (e.g. "edays, ewks"), which effectively impose a 24-hour task  calendar with their own internal conversion factors (24h/ed, 7ed/ew, 30ed/emon). 

Your case is reflecting the default project conversion factor of 8 (working) hours per (working) "day."  Thus,

Construction Activity A Duration = 480 minutes = 8 hrs = 1 "day".

Shutdown Activity B Duration = 1,440 minutes = 24 hrs = 3 "days".


1. Continue using the two calendars and accept what Project tells you, but use "hours" as the duration units for all Shutdown activities.  Then your Shutdown Activity B above would display "Duration = 24 hrs."  If necessary for communications, you can also use a custom text field to convert and display the values you want using a formula.

2. Use elapsed durations for all your shutdown activities.  Then your Shutdown Activity B above would display "Duration = 1 eday."  Issue: since they are presumed to be independent of working time, elapsed durations don't allow for any exceptions to the 24-hour rule, and resource loading can be problematic.

3. Consider using 2 separate projects, one for Construction (Standard Project Calendar, 8h/d) and one for Shutdown (24-hr Project Calendar, 24h/d) Activities.  This works best if there are NO dynamic links between the projects.  (Inter-project links introduce a whole bunch of data-management complications that are best avoided.)

In any case, multiple calendars complicate the interpretation of Total Slack and the "Critical" flag, so describing the "Critical Path" might require a bit of hand-waving.

Good luck, tom