Seeking FOrmula for Calculating Number of Days between 2 Dates

-What formula should I use if I wanted to calculate the number of days between the date in column D and the date in column J once a date is filled into column J and place the answer in column K?

I have been trying to figure this out and am going to continue to do so. I figured I'd ask while I continued working on it.

Thank you in advance for any help.
Tara