Steps to Reproduce

http://zkfiddle.org/sample/3eq32v6/1-ZK-4242

1. Open the datebox
2. Choose 01/01/2443 (AD1900)
3. Open the datebox again

Current Result

It shows "1 ม.ค.2543" (AD2000) in left datebox

Expected Result

It shows "1 ม.ค.2443" (AD1900) in both dateboxes

Debug Information

https://github.com/zkoss/zk/blob/v8.6.2/zul/src/archive/web/js/zul/db/datefmt.js#L242
y = 1900, newY = 43, triggers yy to yyyy
Since ZK stores year in AD internally, it is assumed the year is modified.
It should take this._localizedSymbols.YDELTA into consideration.

Workaround

Use full year format instead (like dd MMM yyyy)