presto-main/src/main/java/io/prestosql/operator/scalar/DateTimeFunctions.java/DateTimeFunctions/truncateTimeWithTimeZone
class DateTimeFunctions:    @Description("truncate to the specified precision")
    @ScalarFunction("date_trunc")
    @LiteralParameters("x")
    @SqlType(StandardTypes.TIME_WITH_TIME_ZONE)
    public static long truncateTimeWithTimeZone(@SqlType("varchar(x)") Slice unit, @SqlType(StandardTypes.TIME_WITH_TIME_ZONE) long timeWithTimeZone)
    {
        long millis = getTimeField(unpackChronology(timeWithTimeZone), unit).roundFloor(unpackMillisUtc(timeWithTimeZone));
        return updateMillisUtc(millis, timeWithTimeZone);
    }