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