fix(timeline): #2795 fix date for custom format function (#2826)

* Fix #2795 issue

lib/timeline/TimeStep.js

@@ -521,6 +521,9 @@ TimeStep.prototype.getLabelMinor = function(date) {
   if (date == undefined) {
     date = this.current;
   }
+  if (date instanceof Date) {
+    date = this.moment(date)
+  }
 
   if (typeof(this.format.minorLabels) === "function") {
     return this.format.minorLabels(date, this.scale, this.step);
@@ -540,7 +543,10 @@ TimeStep.prototype.getLabelMajor = function(date) {
   if (date == undefined) {
     date = this.current;
   }
-  
+  if (date instanceof Date) {
+    date = this.moment(date)
+  }
+
   if (typeof(this.format.majorLabels) === "function") {
     return this.format.majorLabels(date, this.scale, this.step);
   }