Have you noticed that the default date format is in the form mm/dd/yyyy? If you want a different format such as dd/mm/yyyy, you can easily change it.
The date picker date format comes from the site's locale. If you have never changed the site's default locale before, it will be U.S. English:
And the date format will be mm/dd/yyyy as used in U.S.A.:
However if you change the default locale to U.K. English in System & Settings > Multilingual > Multilingual Setup like this:
Then the date format will change to match the locale:
