You need to add a new date and time format that only specifies the date portion under Administer › Site configuration › Date and time › Formats › Add format (the relative path is /admin/settings/date-time/formats/add).
Here are a couple of popular date formats strings according to the PHP date format characters:
m/d/Y
becomes 06/22/2010
l, F j, Y
becomes Tuesday, June 22, 2010
You can either set one of the three default format types (i.e. long, medium, or short) to use your custom format or you can add a new format type. In either case, you would just need to specify the date field to use the format type that is set to your custom date format.