What is your Question?
'Is there a way I could do this via a calculated field? I just need the months of Jan 2017- Nov 2017 to have these FTE values.'
By the sound of what you are saying, I think you are attempting to join the data in the wrong spot i.e. here;
Instead of taking this approach, load each dataset independently (CTRL-D) so that they show up in your data pane as separate data feeds (yes they can even be 2 copies of the same data)
Then use the file menu Data > Edit Relationships to get the following menu up;
Within this menu you can align your two date fields together and the data will blend. Once it is blended, chain link icons will appear in the 2ndary dataset to show you the link points