internal static Task WriteResponseBodyAsync(HttpResponse response, string uid, DateTimeOffset? datetime, TimeSpan? duration, string summary, string description, string location) {
     return response.WriteAsync(
         "BEGIN:VCALENDAR\r\n" +
         "VERSION:2.0\r\n" +
         "BEGIN:VEVENT\r\n" +
         "UID:" + uid + "\r\n" +
         "DTSTART:" + datetime?.ToString(DateTimeFormat) + "\r\n" +
         "DTEND:" + datetime?.Add(duration ?? TimeSpan.Zero).ToString(DateTimeFormat) + "\r\n" +
         "SUMMARY:" + summary + "\r\n" +
         "DESCRIPTION:" + description + "\r\n" +
         "LOCATION:" + location +
         "END:VEVENT\r\n" +
         "END:VCALENDAR\r\n");
 }