The problem is that you are using formfields and content controls in the same document. You shouldn't do that as they weren't designed to work together and trying to do so causes such problems.
As for including months and days, see
Calculate the # Years Months & Days Difference Between Two Dates in my
Microsoft Word Date Calculation Tutorial 'Sticky' thread at the top of the 'Word' forum:
https://www.msofficeforums.com/word/...-tutorial.html
Do read the document's introductory material.