If I understand you correct:
Helper cells E4:E14 where you put the following formula in E4: =MONTH(D4)
and copy down to E14.
Then:
=SUM(1/MMULT((IF((C4:C14="Wheatfields"),E4:E14)=TRANSPOSE (IF((C4:C14="Wheatfields"),E4:E14)))+0,ROW(E4:E14) ^0))-1
This is an array formula so when completing it hold down Ctrl and Shift before pressing Enter.
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