class DateUtil {

    private DateUtil() {
    }

    /**
     * Convenience method that parses a date in yyyyMMdd format and
     * returns a Date object. If the parsing fails, null is returned.
     * @param yyyyMMdd the date string to parse
     * @return a Date object
     * @since 3.0.0
     */
    public static Date parseShortDate(final String yyyyMMdd) {
        final SimpleDateFormat format = new SimpleDateFormat("yyyyMMdd");
        try {
            return format.parse(yyyyMMdd);
        } catch (ParseException e) {
            return null;
        }
    }

    /**
     * Convenience method that parses a date in yyyyMMddHHmmss format and
     * returns a Date object. If the parsing fails, null is returned.
     * @param yyyyMMddHHmmss the date string to parse
     * @return a Date object
     * @since 3.1.0
     */
    public static Date parseDate(final String yyyyMMddHHmmss) {
        final SimpleDateFormat format = new SimpleDateFormat("yyyyMMddHHmmss");
        try {
            return format.parse(yyyyMMddHHmmss);
        } catch (ParseException e) {
            return null;
        }
    }

    /**
     * Convenience method that returns the difference (in days) between
     * two dates.
     * @param start the first date
     * @param end   the second date
     * @return the difference in days
     * @since 3.0.0
     */
    public static long diff(final Date start, final Date end) {
        final long diff = end.getTime() - start.getTime();
        return TimeUnit.DAYS.convert(diff, TimeUnit.MILLISECONDS);
    }


    /**
     * Formats a Date object into ISO 8601 format.
     * @param date the Date object to convert
     * @return a String representation of an ISO 8601 date
     * @since 3.4.0
     */
    public static String toISO8601(final Date date) {<FILL_FUNCTION_BODY>}

    public static Date fromISO8601(final String dateString) {
        if (dateString == null) {
            return null;
        }
        return javax.xml.bind.DatatypeConverter.parseDateTime(dateString).getTime();
    }
}

        final TimeZone tz = TimeZone.getTimeZone("UTC");
        final DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'"); // Quoted "Z" to indicate UTC, no timezone offset
        df.setTimeZone(tz);
        return df.format(date);
    
<no_super_class>