function sd_getHours(tzOffset) 
{
    var hours = tzOffset;
    if ((tzOffset % 30) == 0) 
    {
        var modby = tzOffset % 60;
        if (modby == 30)
            hours -= 30;
        if (modby == -30)
            hours += 30;
        hours /= 60;
        hours = -hours;
        return hours;
    }
    else 
    {
        return NaN;
    }
}