Comparing Data

I have two columns of data that are going to be large ranges.

I need column B to check to see if it's in column A, if it is I then need it to look at the date field in column C and find the difference between column C date and a date in column D, showing the difference between dates in days.

I've attached an example excel spreadsheet.

I've been messing with this trying to do conditional formating, nested IF statements, and I am just lost on how to do this and frustrated.

Can someone please help? I would greatly appreciate it.

Thanks