Interesting use case. The link I've provided shows some methods of getting the users browser format. The methods there are quite reliable. It means you can get exactly the users browser date format. Check it out!

When I'm thinking about your use case the annotation about date format I mentioned would solve the problem for the user. The use case is rare and most of the time browser format will be more accurate than choosing one as a constants.

But your comment prooves that giving user additional way to choose format would improve on the concept further.

The more we talk the more I crave to make a demo 🙂