Tell us how you define a month, then we might be able to suggest a formula.
There are 24 days beyond the 101 months; how did you convert that to 0.856
months?
=DATEDIF(A1,A2,"m")+DATEDIF(A1,A2,"md")/DAY(DATE(YEAR(A2),MONTH(A2),0))
would give 10.857, taking the 28 days in February 2009 (as the 101 months
gets you to 14 Feb 2009) and using that to divide by the 24 odd days.