Projects often require more than one calendar. For example, the office personnel work 8 hours per day, but the field personnel work 10 hours per day (a good reason to be stuck indoors, I suppose). Another reason might be that some activities involve a process that continues around the clock – concrete curing or a test that has to be performed continuously for several days. One of my clients manufacturers most of its components overseas and uses a 7-day calendar for shipping the components to the United States but the fabrication activities are based on a 5-day work week with holidays. So multiple calendars is a sensible approach.
Nevertheless, using more than one calendar on a project is liable to create issues in Primavera P6. Activities will seemingly start or finish on the wrong day. In most situations we would expect that with a Finish to Start relationship the predecessor finishes the day before the successor starts. This is unlikely, however, if the number of hours being worked each day do not match for the predecessor and successor. In the following example I have a predecessor using an 8-hour calendar and a successor using a 10-hour calendar:
The 8-hour task has a start time of 8:00 am and a finish time of 5:00 pm with a 1-hour lunch break. The 10-hour calendar has the same start time but the workday does not end until 7:00 pm. Therefore, the successor can work for two hours on the same day that the predecessor finishes. Since most users do not show the time of day in the date columns it will appear that the relationship between these two activities is something other than Finish to Start.
This problem will reverberate throughout the schedule as one activity after another starts and finishes slightly off the usual time. In order to locate the source of the problem I will sort the activities by the Start date, display the time of day in the date columns and look for the first task that does not start at the usual time.
But how do we stop this from happening in the first place?
The solution is quite simple. Give the 8-hour calendar and the 10-hour calendar the same start and finish times.
I know what you are thinking. The math doesn’t work!
Well, it just takes a bit of fibbing. See, I create an 8-hour calendar with a 2-hour lunch break and a 10-hour calendar with no lunch break. Now both calendars have the same start and finish times. Problem solved. (Our company slogan is “Problems Solved” but I will settle for just one right now).
My solution is displayed below:
The reality is that most users do not want to see the time of day in the date columns so the 2-hour lunch break has no effect whatsoever. I seriously doubt some worker will disappear for two hours because of the CPM schedule. If he or she does, I will buy them lunch!
Some of you might wonder why we should have two different calendars at all if the start and finish times are going to be exactly the same. Well, for resource-loaded schedules it is important to have the hours per day match expectations.
My oil & gas and nuclear clients who schedule by the hour would never submit to this ruse because the time of day will of course be displayed. It is not enough to say an activity starts on, say, Monday, because the activity durations are just a few hours typically. So the next task starts the same day. But for the rest of us this solution works quite well.
I employ a similar strategy for my 7-day calendars. I make sure that the start and finish times match my other calendars. Which means my concrete curing activities typically start at 8:00 am, take a lunch break, and stop at 5:00 pm if my other calendar is an 8-hour calendar. Using a 24-hour calendar would just make things too complicated.
Any questions? Fee free to contact me.