Date format problem

I reopen a ticket as suggested to confirm that the date format problem see #1999966: User 'date' fields added to user from Fields module don't get properly sent to MailChimp still exists with 7.x-4.x and the new mailchimp API

Personally my website use date on the format : dd/mm/yyyy, on mailchimp, I created a merged date field in the same format (dd/mm/yyyy).

But when I register with a date like that : 10/12/2016, I save 12/10/2016 on mailchimp. If I fill 31/10/2016, I get an error as mailchimp understant 10/31/2016 and it says that there is no month with number 31.

So it seems to be the exact same issue that the one I linked above, but on the new mailchimp API so I open a new issue as suggested.