Confusion in Duration concept.

Hi Expert,

I can't understand about duration shown in planning. I have attached a screenshot. for reference. I am calculating duration=end date - start date.
i.e. for id=3 it is showing "164" (not considering both dates in the calculation)
      for id=17 it shows "20" (considering both start and end dates in the calculation).
      for id=286 it shows "1" (considering both start and end dates in the calculation).
      for id=1 it shows "-" 

Please give an understanding of my concern.

Note We have set off days (Saturday & Sunday).
I am using the xlsx file to find the duration (which is also attached for reference)
I have also attached global parameter screen for reference.

 

No, I was just observing progress %, planned work, left work, assigned work & I confused about duration column.

Thanks, I know that duration is calculated using open day only & I am reaching near to solution but here in our Company it is only second and fourth Saturday off. Can We set the calendar in that way? If I can do so It will get exactness in duration.