Tracking open and closed projects - want days open or days to close

Hello. I am looking for assistance with an excel file. Key columns are:
Date Open
Date Closed
Days Open
I would like the Days Open to show:
1. For open items with no closed date: difference b/w date open and today’s date
2. For items with a closed date: difference b/w open date and closed date.
I understand this will require some type of IF statement as well as TODAY and DATEIF functions but cannot get the correct formula.

Some additional info. If have another column that has validation of: open, waiting,closed. When an item is closed "closed" is chosen from drop down and close date is enter into specific column. Therefore I have been using formulas similar to =(IF b2="Closed";DATE d2-c2)...but if not closed then =TODAY-c2.
Sincerely.
DHart