Understanding MSP & Calendar Calculation

[Kathy S]

I have a plan where the duration alternates between days and months
due to federal regulations. This question will focus on the months
duration.
All tasks use the calendar "Weekend Only" which includes weekends,
there are no holidays, the time is 8am-12pm; 1pm-5pm.
Under Tools > Options > Calendar, the days per month is 20.
Both tasks are Fixed Duration and NOT Effort Driven.
Constraint type for both tasks are asap.
There are no resources.
Tasks 1 & 2 both have months duration.
Task 2 has a Task 1FS predecessor.
Task 2 start date is 10/26/08 with 3 mos. duration for a finish date
of 12/24/08.
It is obvious that 3 mos from 10/26/08 is not 12/24/08. The correct
finish date should be 1/26/09.
If I change Under Tools > Options > Calendar, the days per month from
20 to 31, the duration is automatically decreased to 2.9 mos. I then
increase the duration from 2.9 mos to 3 mos and the finish date is
1/26/09. The problem with this change is that all months do not have
just 31 days so other tasks will be off.

Is there a way to dictate specific months have 31 days? What is the
best way to ensure that the finish dates correctly reflect the
duration from the start date? Does anyone have a good explanation as
to how MSP calculates.

Thanks,
Kathy

 

[Dale Howard [MVP]]

Kathy --

Thank you for your clear explanation of your project and what you expect
from Microsoft Project. Unfortunately, the Days Per Month option does not
work the way you want. When you enter 20 days in this field in the Options
dialog, and then enter a Duration in months, such as 3 months, the software
automatically sets the Duration by multiplying 20 times the number of
months. In this case, it would multiple 20 x 3, resulting in a Duration of
60 working days.

Unfortunately, as you can see, the 20 days value is only an approximation of
the number of working days in a typical month. In reality, some months have
20 working days, others have 21, and still others have 22, and it varies
from month to month. So, there's no real way for you to set the Days Per
Month value and then get the results you seek when you enter Duration values
in months. Because of this, your only real solution is to enter the
Duration values in either days or in weeks, but entering them in months will
make your situation nearly impossible to manage. Sorry to deliver the bad
news, but I hope this helps.

 

[Kathy S]

Thank you, Dale. With all the testing I performed, your answer was
what I was experiencing. I appreciate you confirming my test results
and taking the time to respond.

Kathy

 

[Dale Howard [MVP]]

Kathy --

You are more than welcome for the help, my friend!