diff --git a/HISTORY.md b/HISTORY.md
index cc097f1d..141227e7 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -8,6 +8,11 @@ http://visjs.org
 
 - Added German (de) locale. Thanks @Tooa.
 
+### Timeline
+
+- Fixed #1215: inconsistent types of properties `start` and `end` in callback
+  functions `onMove`, `onMoving`, `onAdd`.
+
 
 ## 2015-08-28, version 4.8.0
 
diff --git a/dist/vis.js b/dist/vis.js
index 76dfffa0..9195cc57 100644
--- a/dist/vis.js
+++ b/dist/vis.js
@@ -5,7 +5,7 @@
  * A dynamic, browser-based visualization library.
  *
  * @version 4.8.1-SNAPSHOT
- * @date    2015-08-28
+ * @date    2015-09-04
  *
  * @license
  * Copyright (C) 2011-2015 Almende B.V, http://almende.com
@@ -16734,7 +16734,7 @@ return /******/ (function(modules) { // webpackBootstrap
           item: dragLeftItem,
           initialX: event.center.x,
           dragLeft: true,
-          data: util.extend({}, item.data) // clone the items data
+          data: this._cloneItemData(item.data)
         };
 
         this.touchParams.itemProps = [props];
@@ -16743,7 +16743,7 @@ return /******/ (function(modules) { // webpackBootstrap
           item: dragRightItem,
           initialX: event.center.x,
           dragRight: true,
-          data: util.extend({}, item.data) // clone the items data
+          data: this._cloneItemData(item.data)
         };
 
         this.touchParams.itemProps = [props];
@@ -16752,18 +16752,16 @@ return /******/ (function(modules) { // webpackBootstrap
 
         var baseGroupIndex = this._getGroupIndex(item.data.group);
 
-        this.touchParams.itemProps = this.getSelection().map(function (id) {
+        this.touchParams.itemProps = this.getSelection().map((function (id) {
           var item = me.items[id];
           var groupIndex = me._getGroupIndex(item.data.group);
-          var props = {
+          return {
             item: item,
             initialX: event.center.x,
             groupOffset: baseGroupIndex - groupIndex,
-            data: util.extend({}, item.data) // clone the items data
+            data: this._cloneItemData(item.data)
           };
-
-          return props;
-        });
+        }).bind(this));
       }
 
       event.stopPropagation();
@@ -16805,14 +16803,14 @@ return /******/ (function(modules) { // webpackBootstrap
 
     var newItem = new RangeItem(itemData, this.conversion, this.options);
     newItem.id = id; // TODO: not so nice setting id afterwards
-    newItem.data = itemData;
+    newItem.data = this._cloneItemData(itemData);
     this._addItem(newItem);
 
     var props = {
       item: newItem,
       dragRight: true,
       initialX: event.center.x,
-      data: util.extend({}, itemData)
+      data: newItem.data
     };
     this.touchParams.itemProps = [props];
 
@@ -16851,14 +16849,12 @@ return /******/ (function(modules) { // webpackBootstrap
       }
 
       // move
-      this.touchParams.itemProps.forEach(function (props) {
-        var newProps = {};
+      this.touchParams.itemProps.forEach((function (props) {
         var current = me.body.util.toTime(event.center.x - xOffset);
         var initial = me.body.util.toTime(props.initialX - xOffset);
-        var offset = current - initial;
-
-        var itemData = util.extend({}, props.item.data); // clone the data
+        var offset = current - initial; // ms
 
+        var itemData = this._cloneItemData(props.item.data); // clone the data
         if (props.item.editable === false) {
           return;
         }
@@ -16871,6 +16867,7 @@ return /******/ (function(modules) { // webpackBootstrap
             if (itemData.start != undefined) {
               var initialStart = util.convert(props.data.start, 'Date');
               var start = new Date(initialStart.valueOf() + offset);
+              // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
               itemData.start = snap ? snap(start, scale, step) : start;
             }
           } else if (props.dragRight) {
@@ -16878,6 +16875,7 @@ return /******/ (function(modules) { // webpackBootstrap
             if (itemData.end != undefined) {
               var initialEnd = util.convert(props.data.end, 'Date');
               var end = new Date(initialEnd.valueOf() + offset);
+              // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
               itemData.end = snap ? snap(end, scale, step) : end;
             }
           } else {
@@ -16890,9 +16888,11 @@ return /******/ (function(modules) { // webpackBootstrap
                 var initialEnd = util.convert(props.data.end, 'Date');
                 var duration = initialEnd.valueOf() - initialStart.valueOf();
 
+                // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
                 itemData.start = snap ? snap(start, scale, step) : start;
                 itemData.end = new Date(itemData.start.valueOf() + duration);
               } else {
+                // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
                 itemData.start = snap ? snap(start, scale, step) : start;
               }
             }
@@ -16914,12 +16914,13 @@ return /******/ (function(modules) { // webpackBootstrap
         }
 
         // confirm moving the item
+        itemData = this._cloneItemData(itemData); // convert start and end to the correct type
         me.options.onMoving(itemData, function (itemData) {
           if (itemData) {
             props.item.setData(itemData);
           }
         });
-      });
+      }).bind(this));
 
       this.stackDirty = true; // force re-stacking of all items next redraw
       this.body.emitter.emit('change');
@@ -16959,7 +16960,7 @@ return /******/ (function(modules) { // webpackBootstrap
       var itemProps = this.touchParams.itemProps;
       this.touchParams.itemProps = null;
 
-      itemProps.forEach(function (props) {
+      itemProps.forEach((function (props) {
         var id = props.item.id;
         var exists = me.itemsData.get(id, me.itemOptions) != null;
 
@@ -16977,7 +16978,7 @@ return /******/ (function(modules) { // webpackBootstrap
           });
         } else {
           // update existing item
-          var itemData = util.extend({}, props.item.data); // clone the data
+          var itemData = this._cloneItemData(props.item.data); // convert start and end to the correct type
           me.options.onMove(itemData, function (itemData) {
             if (itemData) {
               // apply changes
@@ -16992,7 +16993,7 @@ return /******/ (function(modules) { // webpackBootstrap
             }
           });
         }
-      });
+      }).bind(this));
     }
   };
 
@@ -17233,7 +17234,7 @@ return /******/ (function(modules) { // webpackBootstrap
       var scale = this.body.util.getScale();
       var step = this.body.util.getStep();
 
-      var newItem = {
+      var newItemData = {
         start: snap ? snap(start, scale, step) : start,
         content: 'new item'
       };
@@ -17241,18 +17242,19 @@ return /******/ (function(modules) { // webpackBootstrap
       // when default type is a range, add a default end date to the new item
       if (this.options.type === 'range') {
         var end = this.body.util.toTime(x + this.props.width / 5);
-        newItem.end = snap ? snap(end, scale, step) : end;
+        newItemData.end = snap ? snap(end, scale, step) : end;
       }
 
-      newItem[this.itemsData._fieldId] = util.randomUUID();
+      newItemData[this.itemsData._fieldId] = util.randomUUID();
 
       var group = this.groupFromTarget(event);
       if (group) {
-        newItem.group = group.groupId;
+        newItemData.group = group.groupId;
       }
 
       // execute async handler to customize (or cancel) adding an item
-      this.options.onAdd(newItem, function (item) {
+      newItemData = this._cloneItemData(newItemData); // convert start and end to the correct type
+      this.options.onAdd(newItemData, function (item) {
         if (item) {
           me.itemsData.getDataSet().add(item);
           // TODO: need to trigger a redraw?
@@ -17419,6 +17421,29 @@ return /******/ (function(modules) { // webpackBootstrap
     return null;
   };
 
+  /**
+   * Clone the data of an item, and "normalize" it: convert the start and end date
+   * to the type (Date, Moment, ...) configured in the DataSet. If not configured,
+   * start and end are converted to Date.
+   * @param {Object} itemData, typically `item.data`
+   * @return {Object} The cloned object
+   * @private
+   */
+  ItemSet.prototype._cloneItemData = function (itemData) {
+    var clone = util.extend({}, itemData);
+
+    // convert start and end date to the type (Date, Moment, ...) configured in the DataSet
+    var type = this.itemsData.getDataSet()._options.type;
+    if (clone.start != undefined) {
+      clone.start = util.convert(clone.start, type && type.start || 'Date');
+    }
+    if (clone.end != undefined) {
+      clone.end = util.convert(clone.end, type && type.end || 'Date');
+    }
+
+    return clone;
+  };
+
   module.exports = ItemSet;
 
 /***/ },
@@ -43054,6 +43079,24 @@ return /******/ (function(modules) { // webpackBootstrap
   exports['en_EN'] = exports['en'];
   exports['en_US'] = exports['en'];
 
+  // German
+  exports['de'] = {
+    edit: 'Editieren',
+    del: 'Lösche Auswahl',
+    back: 'Zurück',
+    addNode: 'Knoten hinzufügen',
+    addEdge: 'Kante hinzufügen',
+    editNode: 'Knoten editieren',
+    editEdge: 'Kante editieren',
+    addDescription: 'Klicke auf eine freie Stelle, um einen neuen Knoten zu plazieren.',
+    edgeDescription: 'Klicke auf einen Knoten und ziehe die Kante zu einem anderen Knoten, um diese zu verbinden.',
+    editEdgeDescription: 'Klicke auf die Verbindungspunkte und ziehe diese auf einen Knoten, um sie zu verbinden.',
+    createEdgeError: 'Es ist nicht möglich, Kanten mit Clustern zu verbinden.',
+    deleteClusterError: 'Cluster können nicht gelöscht werden.',
+    editClusterError: 'Cluster können nicht editiert werden.'
+  };
+  exports['de_DE'] = exports['de'];
+
   // Spanish
   exports['es'] = {
     edit: 'Editar',
diff --git a/lib/timeline/component/ItemSet.js b/lib/timeline/component/ItemSet.js
index be30cf67..c4c11511 100644
--- a/lib/timeline/component/ItemSet.js
+++ b/lib/timeline/component/ItemSet.js
@@ -1160,7 +1160,7 @@ ItemSet.prototype._getGroupIndex = function(groupId) {
         if (groupId == this.groupIds[i])
             return i;
     }
-}
+};
 
 /**
  * Start dragging the selected events
@@ -1174,8 +1174,8 @@ ItemSet.prototype._onDragStart = function (event) {
 
   if (item && item.selected) {
 
-    if (!this.options.editable.updateTime && 
-        !this.options.editable.updateGroup && 
+    if (!this.options.editable.updateTime &&
+        !this.options.editable.updateGroup &&
         !item.editable) {
       return;
     }
@@ -1193,7 +1193,7 @@ ItemSet.prototype._onDragStart = function (event) {
         item: dragLeftItem,
         initialX: event.center.x,
         dragLeft:  true,
-        data: util.extend({}, item.data) // clone the items data
+        data: this._cloneItemData(item.data)
       };
 
       this.touchParams.itemProps = [props];
@@ -1203,7 +1203,7 @@ ItemSet.prototype._onDragStart = function (event) {
         item: dragRightItem,
         initialX: event.center.x,
         dragRight: true,
-        data: util.extend({}, item.data) // clone the items data
+        data: this._cloneItemData(item.data)
       };
 
       this.touchParams.itemProps = [props];
@@ -1216,15 +1216,13 @@ ItemSet.prototype._onDragStart = function (event) {
       this.touchParams.itemProps = this.getSelection().map(function (id) {
         var item = me.items[id];
         var groupIndex = me._getGroupIndex(item.data.group);
-        var props = {
+        return {
           item: item,
           initialX: event.center.x,
           groupOffset: baseGroupIndex-groupIndex,
-          data: util.extend({}, item.data) // clone the items data
+          data: this._cloneItemData(item.data)
         };
-
-        return props;
-      });
+      }.bind(this));
     }
 
     event.stopPropagation();
@@ -1267,14 +1265,14 @@ ItemSet.prototype._onDragStartAddItem = function (event) {
 
   var newItem = new RangeItem(itemData, this.conversion, this.options);
   newItem.id = id; // TODO: not so nice setting id afterwards
-  newItem.data = itemData;
+  newItem.data = this._cloneItemData(itemData);
   this._addItem(newItem);
 
   var props = {
     item: newItem,
     dragRight: true,
     initialX: event.center.x,
-    data: util.extend({}, itemData)
+    data: newItem.data
   };
   this.touchParams.itemProps = [props];
 
@@ -1301,32 +1299,30 @@ ItemSet.prototype._onDrag = function (event) {
     var updateGroupAllowed = me.options.editable.updateGroup;
     var newGroupBase = null;
     if (updateGroupAllowed && selectedItem) {
-        if (selectedItem.data.group != undefined) {
-          // drag from one group to another
-          var group = me.groupFromTarget(event);
-          if (group) {
-            //we know the offset for all items, so the new group for all items
-            //will be relative to this one.
-            newGroupBase = this._getGroupIndex(group.groupId);
-          }
+      if (selectedItem.data.group != undefined) {
+        // drag from one group to another
+        var group = me.groupFromTarget(event);
+        if (group) {
+          //we know the offset for all items, so the new group for all items
+          //will be relative to this one.
+          newGroupBase = this._getGroupIndex(group.groupId);
         }
+      }
     }
 
     // move
     this.touchParams.itemProps.forEach(function (props) {
-      var newProps = {};
       var current = me.body.util.toTime(event.center.x - xOffset);
       var initial = me.body.util.toTime(props.initialX - xOffset);
-      var offset = current - initial;
-
-      var itemData = util.extend({}, props.item.data); // clone the data
+      var offset = current - initial; // ms
 
+      var itemData = this._cloneItemData(props.item.data); // clone the data
       if (props.item.editable === false) {
         return;
       }
 
-      var updateTimeAllowed = me.options.editable.updateTime || 
-                              props.item.editable === true;
+      var updateTimeAllowed = me.options.editable.updateTime ||
+          props.item.editable === true;
 
       if (updateTimeAllowed) {
         if (props.dragLeft) {
@@ -1334,6 +1330,7 @@ ItemSet.prototype._onDrag = function (event) {
           if (itemData.start != undefined) {
             var initialStart = util.convert(props.data.start, 'Date');
             var start = new Date(initialStart.valueOf() + offset);
+            // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
             itemData.start = snap ? snap(start, scale, step) : start;
           }
         }
@@ -1342,6 +1339,7 @@ ItemSet.prototype._onDrag = function (event) {
           if (itemData.end != undefined) {
             var initialEnd = util.convert(props.data.end, 'Date');
             var end = new Date(initialEnd.valueOf() + offset);
+            // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
             itemData.end = snap ? snap(end, scale, step) : end;
           }
         }
@@ -1355,18 +1353,20 @@ ItemSet.prototype._onDrag = function (event) {
               var initialEnd = util.convert(props.data.end, 'Date');
               var duration  = initialEnd.valueOf() - initialStart.valueOf();
 
+              // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
               itemData.start = snap ? snap(start, scale, step) : start;
               itemData.end   = new Date(itemData.start.valueOf() + duration);
             }
             else {
+              // TODO: pass a Moment instead of a Date to snap(). (Breaking change)
               itemData.start = snap ? snap(start, scale, step) : start;
             }
           }
         }
       }
 
-      var updateGroupAllowed = me.options.editable.updateGroup || 
-                               props.item.editable === true;
+      var updateGroupAllowed = me.options.editable.updateGroup ||
+          props.item.editable === true;
 
       if (updateGroupAllowed && (!props.dragLeft && !props.dragRight) && newGroupBase!=null) {
         if (itemData.group != undefined) {
@@ -1381,12 +1381,13 @@ ItemSet.prototype._onDrag = function (event) {
       }
 
       // confirm moving the item
+      itemData = this._cloneItemData(itemData);  // convert start and end to the correct type
       me.options.onMoving(itemData, function (itemData) {
         if (itemData) {
           props.item.setData(itemData);
         }
       });
-    });
+    }.bind(this));
 
     this.stackDirty = true; // force re-stacking of all items next redraw
     this.body.emitter.emit('change');
@@ -1445,7 +1446,7 @@ ItemSet.prototype._onDragEnd = function (event) {
       }
       else {
         // update existing item
-        var itemData = util.extend({}, props.item.data); // clone the data
+        var itemData = this._cloneItemData(props.item.data); // convert start and end to the correct type
         me.options.onMove(itemData, function (itemData) {
           if (itemData) {
             // apply changes
@@ -1461,7 +1462,7 @@ ItemSet.prototype._onDragEnd = function (event) {
           }
         });
       }
-    });
+    }.bind(this));
   }
 };
 
@@ -1708,7 +1709,7 @@ ItemSet.prototype._onAddItem = function (event) {
     var scale = this.body.util.getScale();
     var step = this.body.util.getStep();
 
-    var newItem = {
+    var newItemData = {
       start: snap ? snap(start, scale, step) : start,
       content: 'new item'
     };
@@ -1716,18 +1717,19 @@ ItemSet.prototype._onAddItem = function (event) {
     // when default type is a range, add a default end date to the new item
     if (this.options.type === 'range') {
       var end = this.body.util.toTime(x + this.props.width / 5);
-      newItem.end = snap ? snap(end, scale, step) : end;
+      newItemData.end = snap ? snap(end, scale, step) : end;
     }
 
-    newItem[this.itemsData._fieldId] = util.randomUUID();
+    newItemData[this.itemsData._fieldId] = util.randomUUID();
 
     var group = this.groupFromTarget(event);
     if (group) {
-      newItem.group = group.groupId;
+      newItemData.group = group.groupId;
     }
 
     // execute async handler to customize (or cancel) adding an item
-    this.options.onAdd(newItem, function (item) {
+    newItemData = this._cloneItemData(newItemData);     // convert start and end to the correct type
+    this.options.onAdd(newItemData, function (item) {
       if (item) {
         me.itemsData.getDataSet().add(item);
         // TODO: need to trigger a redraw?
@@ -1901,4 +1903,27 @@ ItemSet.itemSetFromTarget = function(event) {
   return null;
 };
 
+/**
+ * Clone the data of an item, and "normalize" it: convert the start and end date
+ * to the type (Date, Moment, ...) configured in the DataSet. If not configured,
+ * start and end are converted to Date.
+ * @param {Object} itemData, typically `item.data`
+ * @return {Object} The cloned object
+ * @private
+ */
+ItemSet.prototype._cloneItemData = function (itemData) {
+  var clone = util.extend({}, itemData);
+
+  // convert start and end date to the type (Date, Moment, ...) configured in the DataSet
+  var type = this.itemsData.getDataSet()._options.type;
+  if (clone.start != undefined) {
+    clone.start = util.convert(clone.start, type && type.start || 'Date');
+  }
+  if (clone.end != undefined) {
+    clone.end = util.convert(clone.end , type && type.end || 'Date');
+  }
+
+  return clone;
+};
+
 module.exports = ItemSet;
diff --git a/test/timeline_groups.html b/test/timeline_groups.html
index aac1c2ca..4df1b433 100644
--- a/test/timeline_groups.html
+++ b/test/timeline_groups.html
@@ -71,7 +71,9 @@
   }
 
   // create a dataset with items
-  var items = new vis.DataSet();
+  var items = new vis.DataSet({
+    type: {start: 'Moment', end: 'Moment'}
+  });
   for (var i = 0; i < itemCount; i++) {
     var start = now.clone().add(Math.random() * 200, 'hours');
     var end = Math.random() > 0.5 ? start.clone().add(24, 'hours') : undefined;
@@ -117,6 +119,7 @@
     },
 
     onMove: function (item, callback) {
+      console.log('onMove', item)
       if (confirm('Do you really want to move the item to\n' +
           'start: ' + item.start + '\n' +
           'end: ' + item.end + '?')) {