Global calendars are great, because they are available for multiple project schedules. But this advantage can be a double edged sword. Let’s take a look.
The Naval Facilities Engineering Command (NAVFAC) Integrated Master Schedule (IMS) review worksheet requires that calendars be defined at the project level. The outstanding question: why does the NAVFAC schedule checklist favor calendars defined at the project level?
Why not recommend global calendars, which can be leveraged for use on multiple project schedules? The short answer is that the global calendar’s broad availability is both a positive and a negative.
Global and Project Calendars
This salient tip article demonstrates why schedulers should define their calendars at the project level, and not the global level.
We have in Figure 1 one of two demonstration projects.
This is a month long Piping Inspection and Repair project starting April 1st. Note the 5×8 calendar in the activities table. Now we open another project named Pipe Repair and Improve, Figure 2.
Again, note the same 5×8 calendar and April 1st start date.
We continue and select Enterprise | Calendars to bring up the Calendars dialog, Figure 3.
First, note the default calendar is 5×8 Standard, which is not the calendar assigned to our two demonstration projects. You cannot delete the default calendar, but you should be able to delete other non-default calendars.
For demonstration purposes we try to delete the 5×8 calendar assigned to both our demonstration projects. When we try to delete the 5×8 calendar we get the message “calendar 5×8 cannot be deleted because it is used by other projects”. This is good. We would not want to delete a calendar used by the currently open project and other projects.
So it is definitive; calendars used by more than one project cannot be deleted. But can a calendar used by more than one project be changed? Let’s investigate. We again select Enterprise | Calendars to activate the calendars dialog. This time we choose the 5×8 calendar and do not try to delete it; we try to modify it, Figure 4.
In Figure 5 we adjust the 5×8 global calendar by adding a one week spring break.
We simply make April 8th through 12th non-work days, again, Figure 5. We continue and select OK to confirm our calendar updates.
When we exit the calendar dialog, we, again, view our second demonstration project, Pipe Repair and Improve. It looks the same, despite having added the one week spring break. But watch what happens when we recalculate the schedule, Figure 6.
All activities in the second week, Figure 7, are delayed one week for spring break.
Okay, that makes sense. We modified the calendar to include the spring break non-work week. Let’s continue and reopen the first demonstration project, Piping Inspection & Repair. When we do, we observe it looks the same as previous. Before we draw any conclusions we, again, recalculate the schedule, Figure 8.
Again, we observe in Figure 9 that all activities during the second workweek are delayed for spring break.
Now what can we conclude? Fist, calendars that have been assigned to more than one project cannot be deleted. Great! We know that from the warning message, Figure 3. Second, calendars assigned to more than one project, however, can be modified.
So changing a global calendar assigned to multiple projects will change all the schedules, accordingly. Schedulers that are not aware of this global calendar trait can unwittingly make collateral changes to a multitude of projects simply by adjusting the associated global calendar. Not good. For this reason, most scheduling guidelines recommend defining calendars at the project level. This way any change you make to the calendar only impacts the schedule assigned that project calendar.
One last point in passing. Note that when we make 5×8 the default calendar the shading on the Gantt chart matches, accordingly, to darken the spring break week, Figure 10.
The lesson is that the default calendar in the database determines the Gantt chart shading for all the database schedules, so you want to be aware of your default calendar. And compare its Gantt chart shading to that of your schedule’s project calendar.
In particular, before printing the Gantt chart for stakeholder review. No, project calendars cannot be default calendars. You, again, should note the database default calendar for shading. Everything else will be effected by the project calendar.
Global and Project Calendars Summary
Scheduling guidelines come with much thought and foresight. And there is a reason most scheduling guidelines recommend defining calendars at the project level, and not the global level. Any modification to a global calendar automatically updates all schedules assigned that global calendar.
A project calendar modification only effects the one schedule assigned to it. Therefore, it is better not to try and replicate a calendar across a multitude of schedules by making it a global calendar.
If you want to leverage your effort in creating a calendar for additional schedules we recommend selecting that calendar in the “select calendar to copy from” dialog that pops up whenever you add a new project calendar in Primavera P6 Professional. This makes the project calendar a carbon copy of the “copy from” calendar, which you can change, as appropriate, for your project schedule.