Steps to Reproduce

run following datebox with 8.6.0.FL.20181023

<datebox lenient="true" strictDate="true" format="dd.MM.yyyy"></datebox>

input the date: 31.11.2018

Current Result

Datebox will switch to 01.12.2018

Expected Result

Datebox should reject the date (strictDate=true)

Debug Info

strictDate is evaluated before formatting.
If the formatted date is different than the input string, the result date may rejected while it is valid, or not rejected while it is not valid.

strictDate:
https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/db/datefmt.js#L325

formatting:
https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/db/datefmt.js#L389-L396

Could be improved by checking the same values checked during strict mode checks here:
https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/db/datefmt.js#L389-L396

Root Cause

Workaround