如何将Java时间戳转换为不同时区？

public static Calendar convertToGmt(Calendar cal) {

    Date date = cal.getTime();
    TimeZone tz = cal.getTimeZone();

    // Milliseconds since epoch GMT
    long msFromEpochGmt = date.getTime();

    // Current offset from GMT at current date
    int offsetFromUTC = tz.getOffset(msFromEpochGmt);

    // Create a GMT calendar, set time, and add offset
    Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCal.setTime(date);
    gmtCal.add(Calendar.MILLISECOND, offsetFromUTC);

    return gmtCal;
}