presto-main/src/main/java/io/prestosql/util/DateTimeZoneIndex.java/DateTimeZoneIndex/extractZoneOffsetMinutes
class DateTimeZoneIndex:    public static int extractZoneOffsetMinutes(long dateTimeWithTimeZone)
    {
        short zoneKey = unpackZoneKey(dateTimeWithTimeZone).getKey();

        if (FIXED_ZONE_OFFSET[zoneKey] == VARIABLE_ZONE) {
            return DATE_TIME_ZONES[zoneKey].getOffset(unpackMillisUtc(dateTimeWithTimeZone)) / 60_000;
        }
        else {
            return FIXED_ZONE_OFFSET[zoneKey];
        }
    }