Comparing Date Diff between Two Date fields but need to know if Met OnTime

Anonymous , You can have new columns like

KPI Status = if([Close Date Month] >=[Expiration Date Month],"KPI Met (OnTime)", " KPI Met (not OnTIme)")

Diff when not met = if([Close Date Month] >=[Expiration Date Month],blank() , datediff([Close Date Month],[Expiration Date Month],day))

or  one combined  

combined = if([Close Date Month] >=[Expiration Date Month],"KPI Met (OnTime)", " KPI Met (not OnTIme) ") & if([Close Date Month] >=[Expiration Date Month],blank() , datediff([Close Date Month],[Expiration Date Month],day)) & " months behind"