The date format of a promotion depends on the language set for the promotion. If your promotion language is set to “Automatic”, it means that the interface of the application is displayed depending on the language of the user viewer on Facebook. This means that if the user who visits your promotion has the language set to “English US”, then the application will show the date in format mm/dd/yyy.
In order to change the date format, you should set the type of English you want to force for your promotion, by accessing the basic configuration section of the promotion, where you’ll have the language selector:
You can edit your promotion from your administration panel, which you can access by following this link: http://admin.easypromosapp.com