You actually have to reinstall firefox using a different download to get `new Date(dateString)` to use a different date format (i.e. non-US).
https://jqueryvalidation.org/date-method/
This method should not be used, since it relies on thenew Dateconstructor, which behaves very differently across browsers and locales. UsedateISOinstead or one of the locale specific methods (in localizations/ and additional-methods.js).
So if you're using chocolatey or boxstarter like I am (my boxstarter script) you need to add the locale flag: https://chocolatey.org/packages/firefox
choco install Firefox -packageParameters "l=en-GB".Reinstalling the package with --force is sufficient to change it.
It would seem chrome has no way change the format from en-US.
To be clear the correct solution is to do as the documentation says and not use that method, however it's a bit mean having a pitfall like that in the API. And sometimes you just have to work with the code you have...