In design mode, select the date picker. Press F4 to get the Properties window. Scroll all the way to the top of the Props window. There's one called "Custom" click the "..." button that's there (you may have to actualy select the property before it shows I think).

A new dialog screen should pop up. There's a checkmark that controls if the date shoudl be displayed or not. Clear it. Set any other props you want. Click OK (or it may be close, I can't remember off the top of my head.) Yuo should be OK after that. If you are using the .Value property to get the date selected, it may still contain the time, just simply use the Format function to return only the date.

Tg