SignupController.groovy

def save = {
  def sdfh = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss")
  params.signupdate = sdfh.parse(params.signupdate)

  def u = new Signup()
  u.properties = params

  if (u.save()) {
    render u as XML
  } else {
    render u.errors as XML
  }
}