fix(#366): always use Dates not numbers for custom bar timestamps (#…

…368)

Co-authored-by: Yotam Berkowitz <[email protected]>

lib/timeline/Core.js

@@ -659,7 +659,7 @@ class Core {
    */
   addCustomTime(time, id) {
     const timestamp = time !== undefined
-        ? util.convert(time, 'Date').valueOf()
+        ? util.convert(time, 'Date')
         : new Date();
 
     const exists = this.customTimes.some(customTime => customTime.options.id === id);