Deserialize old Microsoft Json date

gistfile1.txt

private static class MicrosoftJsonDate {
    private static final int DATE_GMT0_GROUP = 1;
    private static final int OFFSET_GROUP = 2;
    private static Pattern json_date_pattern
            = Pattern.compile("^\\/Date\\(([-]?\\d+)([-\\+]([01]\\d|2[0-3])[0-5]\\d)?\\)\\/$");

    private static HashMap<String, Integer> offsetInMillisMap
            = new HashMap<>();

    public static Date deserialize(String json_date) throws IllegalStateException {
        Matcher m = json_date_pattern.matcher(json_date);

        if(m.find()) {
            Long date = Long.parseLong(m.group(DATE_GMT0_GROUP)) + getOffsetInMillis(m.group(OFFSET_GROUP));
            return new Date(date);
        }
        else {
            throw new IllegalStateException("The value '" + json_date + "' is an invalid Microsoft JSON date.");
        }
    }

    private static Integer getOffsetInMillis(String offset) {
        if (offset == null || offset.length() == 0) {
            return 0;
        }
        else if (offsetInMillisMap.containsKey(offset)) {
            return offsetInMillisMap.get(offset);
        }
        else {
            Integer offsetInMillis = TimeZone
                    .getTimeZone("GMT" + offset)
                    .getRawOffset();

            offsetInMillisMap.put(offset, offsetInMillis);

            return offsetInMillis;
        }
    }
}