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var MyModel = Backbone.Model.extend({
  initialize: (attrs){
    var self = this;
    self.updateDate();
    self.bind('change:date', function(){
      self.updateDate();
    });
  },
  updateDate: function(){
    var date = this.get('date');
    if(_.isString(date)){
      this.set('date', new Date(date));
    }
  }
});

or

var MyModel = Backbone.Model.extend({
  set: function() {
    Backbone.Model.prototype.set.apply(this, arguments);
    this.updateDate();
  },
  updateDate: function(){
    var date = this.get('date');
    if(_.isString(date)){
      this.set('date', new Date(date));
    }
  }
});