//  ------------------------------------------------------------------
//  getDateTimeGmt  : Returns a datetime in GMT from a timezoneId, date and time object
//  ------------------------------------------------------------------
    public static Datetime getDateTimeGmt(String tzId, Date d, Time t) {
        DateTime    res;
        if (tzId == null || d == null || t == null) return res;
        Timezone    tz          = Timezone.getTimeZone(tzId);
        DateTime    dt          = DateTime.newInstanceGmt(d,t);
        Integer     offsetMs    = tz.getOffset(dt);                 // #ms offset from datetime in Tz to GMT
        Integer     offsetMins  = offsetMs / (1000 * 60);
        res = dt.addMinutes(-1 * offsetMins);

        return res;
    }