How to ensure a DateTime or Timestamp type is returned

Hi, 

I am using the Decode Datebase Function to return a datetime when a task has a status of "Completed". 
It looks like this:

However, when I use the above approach the return type is string and not a timestamp.

The reason I want a timestamp for this value is so the user can set the criteria/param before generating the report easily by selecting a date from the calendar picker (like in the image below) instead of entering free text.