diff --git a/src/utils.js b/src/utils.js
index 7b1712bda..6ef4ce629 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -16,11 +16,12 @@ const padZoneStr = (instance) => {
 
 const monthDiff = (a, b) => {
   // function from moment.js in order to keep the same result
+  if (a.date() < b.date()) return -monthDiff(b, a)
   const wholeMonthDiff = ((b.year() - a.year()) * 12) + (b.month() - a.month())
-  const anchor = a.clone().add(wholeMonthDiff, C.M)
+  const anchor = a.add(wholeMonthDiff, C.M)
   const c = b - anchor < 0
-  const anchor2 = a.clone().add(wholeMonthDiff + (c ? -1 : 1), C.M)
-  return Number(-(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) :
+  const anchor2 = a.add(wholeMonthDiff + (c ? -1 : 1), C.M)
+  return +(-(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) :
     (anchor2 - anchor)))) || 0)
 }