index.js

module.exports = timestamps;

function timestamps(createdAtField, updatedAtField) {
  if(typeof createdAtField === 'function') {
    return plugin('createdAt', 'updatedAt', createdAtField);
  }
  else {
    createdAtField = createdAtField || 'createdAt';
    updatedAtField = updatedAtField || 'updatedAt';
    return plugin.bind(null, createdAtField, updatedAtField);
  }
}

function plugin(createdAtField, updatedAtField, Model) {
  Model.attr(createdAtField, { type: 'date' });
  Model.attr(updatedAtField, { type: 'date' });

  Model.on('saving', function(instance, done){
    var now = new Date();

    if(instance.isNew()) {
      instance[createdAtField](now);
    }

    instance[updatedAtField](now);

    var set;
    if ('string' === typeof instance.get(createdAtField)) {
      set = {};
      set[createdAtField] = new Date(instance.get(createdAtField));
      instance.set(set);
    }

    if(done) done();
  });

  Model.on('initializing', function(instance, attrs) {
    if ('string' === typeof attrs[createdAtField]) {
      attrs[createdAtField] = new Date(attrs[createdAtField]);
    }
    if ('string' === typeof attrs[updatedAtField]) {
      attrs[updatedAtField] = new Date(attrs[updatedAtField]);
    }
  });
}