Calc difference between two dates in days

Hello,

I've been striking out on what I believe is something simple. I have two date fields in my table. They are both formatted as a date. I want to create a column that shows the #days difference between the two days. I checked other solutions and many of them suggested using DateDiff but I havent been able to get it to work. 

Table: Main Table

Field1: Date Completed

Field 2: Date Received

I want to be able to get the # days between Field 2-Field 1.