/**
   * Returns the time that this cookie expires, in the same format as [System.currentTimeMillis].
   * This is December 31, 9999 if the cookie is not [persistent], in which case it will expire at the
   * end of the current session.
   *
   * This may return a value less than the current time, in which case the cookie is already
   * expired. Webservers may return expired cookies as a mechanism to delete previously set cookies