To set your own date format for the community go to My settings -> Preferences -> Display. You can reach My settings from the drop down menu opened by clicking your avatar in the top right corner of any community page.
Once there you can enter your own personalized date format using upper case MM for months lower case dd for day and lower case yyyy for year.
Here are some examples:
You can use any date pattern supported by the Java date and time parser. You can find more information here SimpleDateFormat (Java Platform SE 6) (oracle.com).
Once you set your own format it will override the community default. To clear your format and restore the default leave the Date display format field blank and click save.