diff --git a/lib/index.js b/lib/index.js
index 466ec72..b9688da 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -276,11 +276,15 @@ GitStats.ansiCalendar = function (data, callback) {
       , strYear = ""
       , w = 0
       , d = 0
+      , dataClone = {
+            start: data.start ? Moment(data.start.format(DATE_FORMAT), DATE_FORMAT) : null
+          , end: data.end ? Moment(data.end.format(DATE_FORMAT), DATE_FORMAT) : null
+        }
       ;
 
     GitStats.calendar(data, function (err, cal) {
         if (err) { return callback(err); }
-        GitStats.iterateDays(function (cDay, mDay) {
+        GitStats.iterateDays(dataClone, function (cDay, mDay) {
             sDay = mDay.format("ddd");
             if (mDay.format("D") === "1") {
                 months.push(mDay.format("MMM"));