Steps to Reproduce

open in zkfiddle

<zk>
    <datebox format="dd/MM/yyyy"/>
    <datebox format="dd/MM/yy"/>
</zk>

1. Open datebox to choose Jan 1st, 1900
2. Open datebox again

Current Result

dd/MM/yyyy => 1st Jan, 1900
dd/MM/yy => 1st Jan, 2000

Expected Result

dd/MM/yyyy => 1st Jan, 1900
dd/MM/yy => 1st Jan, 1900

Since the date is by choice, not by inputting. Should have confidence about the year.

Debug Information

https://github.com/zkoss/zk/blob/v8.6.1/zul/src/archive/web/js/zul/db/Datebox.js#L766
https://github.com/zkoss/zk/blob/v8.6.1/zul/src/archive/web/js/zul/inp/InputWidget.js#L806
When a date is chosen, it will call updateChange_ which is called _validate to coerce from text

Workaround

Avoid using yy only in format.