Date Formatting

I have a workflow that has three columns.  Effect Date (2016-01-05),  First of Current Month (2016-02-01) and 185 Days (2015-07-31).

All of these are in the date format.

I try to format any of the dates so it would be 01/05/2016. 

Here is the formula I have been using. DateTimeFormat([EFFECT DATE],"%m/%d/%y")

No matter what I try, I keep getting NULL.

Any ideas?