namespace Xmpp.Xep.DateTimeProfiles {

public DateTime? parse_string(string time_string) {
    Regex DATETIME_REGEX = new Regex("""^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(\.(\d{3}))?(Z|((\+|\-)(\d{2})(:(\d{2}))?))$""");
    MatchInfo match_info;
    if (DATETIME_REGEX.match(time_string, RegexMatchFlags.ANCHORED, out match_info)) {
        int year = int.parse(match_info.fetch(1));
        int month = int.parse(match_info.fetch(2));
        int day = int.parse(match_info.fetch(3));
        int hour = int.parse(match_info.fetch(4));
        int minute = int.parse(match_info.fetch(5));
        int second = int.parse(match_info.fetch(6));
        DateTime datetime = new DateTime.utc(year, month, day, hour, minute, second);
        if (match_info.fetch(9) != "Z") {
            char plusminus = match_info.fetch(11)[0];
            int tz_hour = int.parse(match_info.fetch(12));
            int tz_minute = int.parse(match_info.fetch(13));
            if (plusminus == '-') {
                tz_hour *= -1;
                tz_minute *= -1;
            }
            datetime.add_hours(tz_hour);
            datetime.add_minutes(tz_minute);
        }
        return datetime;
    }
    return null;
}

public string to_datetime(DateTime time) {
    return time.to_utc().format("%Y-%m-%dT%H:%M:%SZ");
}

}