presto-main/src/main/java/io/prestosql/operator/scalar/DateTimeFunctions.java/DateTimeFunctions/fromISO8601Date
class DateTimeFunctions:    @ScalarFunction("from_iso8601_date")
    @LiteralParameters("x")
    @SqlType(StandardTypes.DATE)
    public static long fromISO8601Date(ConnectorSession session, @SqlType("varchar(x)") Slice iso8601DateTime)
    {
        DateTimeFormatter formatter = ISODateTimeFormat.dateElementParser()
                .withChronology(UTC_CHRONOLOGY);
        DateTime dateTime = parseDateTimeHelper(formatter, iso8601DateTime.toStringUtf8());
        return MILLISECONDS.toDays(dateTime.getMillis());
    }