Halfway point between 2 dates in a formula field

Halfway point between 2 dates in a formula field

Hi 

I am trying to create a formula based on a halfway point between 2 dates, I have tried the following but am getting a syntax error on both issues..

Date_Span = (${MODULE.Start Date},${Module.End Date});
Number_Of_Days = (Date_Span  /  (1000  *  60  *  60  *  24  ));
Half_Days = (Number_Of_Days /2).round(0);
HalfwayPoint=  ${Module.Start Date.addDay(Half_Days);


or Halfway point = (${MODULE.Start Date},${Module.End Date}/2)

Not sure where Ive gone wrong?