With Locale settings you can make sure that your date/time, calendar, and number fields are in the correct format for your geographical location. Editing your Locale settings is a simple task that just takes a few steps:
There are 2 places that the date format is set up. One for the user and one for company wide.
To change this company wide you would go to Setup / Administration Setup / Company Profile / Company Information / then change the Default Locale to what ever suits your needs (in this case we use English (United Kingdom)).
This is the company wide settings and each user will set up their setting correctly if they have it changed. This is under Setup / Personal Setup / Personal information then edit the Locale to English (United Kingdom).