Unable to reproduce. What is the date entered and the exact error message?

I narrowed it down a bit more. It seems this error only appeared when my backend language is set to Dutch. When I set it to the default English the problem does not exist. I have some screenshots to illustrate:

https://inxpactdrive.nl/s/npXCWKOU5hvHBzl

Not sure how or why a backend language would cause this.

I tried to disable all admin language overrides. That did not help.

Well, the problem is solved for me by adding this to the language overide for the backend Dutch language:

DATE_FORMAT_CALENDAR_DATE="%d-%m-%Y"
DATE_FORMAT_CALENDAR_DATETIME="%d-%m-%Y %H:%M:%S"
DATE_FORMAT_FILTER_DATE="d-m-Y"
DATE_FORMAT_FILTER_DATETIME="d-m-Y H:i:s"

Not the date is displayed in the Dutch format (better for us...) and the article gets saved OK, And as far as I can see this has no further impact on the site. So this solves this issue for me.

But maybe this problem should not occur at all? I will leave this assessment to others for now.
Thanks for listening so far ;-)

@jjnxpct can you please get in touch with one Dutch-Translator at https://docs.joomla.org/JDOC:Documentation_Translators#nl_-_Nederlands_-_Dutch?

NOTE: not the first time there has been an issue with the dutch date format iirc - maybe @Bakual can confirm

@jjnxpct There is small typo in Dutch language package for Joomla! 3.9.0 both site and admin in nl-NL.ini there is DATE_FORMAT_FILTER_DATE="y-m-d" instead of DATE_FORMAT_FILTER_DATE="Y-m-d".

Dutch TT can be reached at
https://forum.joomla.org/memberlist.php?mode=viewprofile&u=129749

@wojsmol Excelent find. Thanks!

I noticed the same error in these language strings:
DATE_FORMAT_LC4="y-m-d" (admin nl-NL.ini)
DATE_FORMAT_JS1="y-m-d" (site nl-NL.ini)

cc @MartijnMaandag (this is Dutch TT github nick) Please check Dutch language pack regarding date formats.

I'm going to close this issue then as it is indeed a bug in the language.

I think @brianteeman was referring to the PR #22373 which tries to solve an issue that is already solved if the language pack would be corrected (from how I understand that PR) ?

The date format in the Dutch language files is like this for years. I changed it and am testing this with @jjnxpct
When it works I will release a new Dutch package.

@MartijnMaandag I tested the new language files you send me. I cleared all the overrides and there are no issues as far as I can tell. The problem I had described in this thread is solved. Thanks!

I'm closing this issue then.