Would it be possible to prepare some sample data and attach tableau workbook.
Sorry. Thought I had. Attached now.
You may be able to do this without table calculations. Here I create new two calculated fields:
[Elapsed Time]=MAX([End Date])-MIN([Start Date])
[Completion Month]=DATENAME('month', MAX([End Date]))
You still need to put the [Company] field in the view, though with Tableau 9.0 you may be able to use the new LOD calculations to include [Company] implicitly though I don't know how you want to display multiple projects completed in the same month.
Image below and modified 8.2 workbook attached.
Stage Test ZL.twbx 14.8 KB