Steps to Reproduce

display a datebox using the Portuguese locale in 'medium' or 'long' format

<datebox locale="pt" format="medium" onCreate="self.setValue(new Date());" width="300px"/>
<separator/>
<datebox locale="pt" format="long" onCreate="self.setValue(new Date());" width="300px"/>

Current Result

(depending on the JDK version outputs may vary due to differenct CLDR versions)

JDK 11 result (2021-10-14):

Expected Result

medium: "14 de out de 2021"
long: "14 de outubro de 2021"

Debug Information

the correct date formats are provided by the server :
medium: "d 'de' MMM 'de' y"
long: "d 'de' MMMM 'de' y"

notice the single quotes around the word 'de'

When formatting the date zk.fmt.DateFormat ignores the single quotes and treats the character 'd' and 'e' as date pattern tokens (d = day of month / e = unknown resulting in a '1' https://github.com/zkoss/zk/blob/v9.6.0.1/zul/src/archive/web/js/zul/db/datefmt.js#L567-L571) hence composes the formatted date incorrectly.

Also updating the date via calendar or typing a valid date doesn't update the component (assuming the parser also fails to treat quoted characters accordingly - https://docs.oracle.com/javase/8/docs/api/java/text/SimpleDateFormat.html)

Workaround

avoid quoted characters in date formats