Backbone mixin that maintains a timestamp for the last successful "sync" event. This can be used on both Models and Collections.

lastsync.mixin.js

// Last Sync timestamp mixin. Works for Models and Collections.
var LastSync = {
  // Update lastSync with current timestamp.
  _timestampSync: function() {
    this.lastSync = new Date();
  },
  // Note: Call LastSync.initialize.apply(this) from the Model/Collection .initialize.
  initialize: function() {
    // Setup lastSync to timestamp sync events.
    this.lastSync = null;
    // Events.
    this.on('sync', this._timestampSync, this);
  }
};