Count the number of whole calandar months in a date range

Cool examples! All your scenarios is working with this formula. Try it out.

Formula:

=O12/12*((IF(AND(G12>EOMONTH(F12,0)+1,F12<EOMONTH(G12,-1),EOMONTH(F12,0)<EOMONTH(G12,-1)),DATEDIF(EOMONTH(F12,0)+1,EOMONTH(G12,-1),"M")+1,0))+(IF(AND(MONTH(F12)=MONTH(G12),YEAR(F12)=YEAR(G12)),(DATEDIF(F12,G12,"D")+1)/DAY(EOMONTH(F12,0)),((IF(DAY(F12)=1,1,((DATEDIF(F12,EOMONTH(F12,0),"D")+1)/DAY(EOMONTH(F12,0)))))+(IF(DAY(G12)=DAY(EOMONTH(G12,0)),1,((DATEDIF(EOMONTH(G12,-1)+1,G12,"D")+1)/DAY(EOMONTH(G12,0)))))))))