Adding to an IF Statement or change to IFS statement

Hi,

I have an IF formula to work out the numbers of days late (or early) an order is compared to the first date I confirmed back to the customer.
I ran into a problem where an order has a confirmed date but no current in stock date. Numerous reasons why this happens but I can't figure out how to either nest my current IF statement or change it to an IFS statement to include a condition that if there is a First confirmed date (column D) but no Current in stock (column C) that it should return a blank.

Any help on this is much appreciated. Sample book attached.