Webform - how do I change dateformat in mail
Try go to admin/config/regional/date-time and set your site's Medium date type format.

Also, if the Medium format doesn't include at least a month, day, and year field, Webform will default to the American format of MM/DD/YYYY.

Webform uses the "medium" in e-mails and viewing submissions, though everywhere else (like a displaying the date component) it uses short for the order