As of this moment, in Confluence, formatting the Date Picker depends on the user's language setting. There is a feature request to make it independent, so you can apply the international standard YYYY-MM-DD, or any other standard, independent of language.
If you agree, please up-vote this request:
https://jira.atlassian.com/browse/CONFSERVER-35168
Yes, we have more urgent things, but this is one small thing that wastes time and if it was resolved, we'd have just that much more time to invest in more urgent, more important things.
