Date Formatting for formula

Date Formatting for formula

I currently have a Start_Date field on a custom form.  The user inputs the date and another field, End_Date is a formula calculated a week after the start date.  The code to do which is simple enough.

(Start_Date.addDay(7)).toDate("dd-MM-yyyy")

 
It works fine aside form displaying with the time.
All records have the date followed by 00:00:00.  I don't want it to display time.  I'm sure its something simple that I'm just not seeing and thank you in advance.
-Ian