Zend certified PHP/Magento developer

How to calculate how many days have passed based on two cells values

I am trying to calculate how many days have passed upon receiving a PO after an NTP has been provided. The part I’m struggling with is that sometimes based on an entry there could be a blank in either the NTP Date or the PO Date and I need to calculate a different formula based on that.

Here's my table

Ideally, I would like it to look something like this in the PO Delay column,

Calculate [PO DATE]-[NTP Date], If [PO DATE] is blank calculate =TODAY()-[NTP DATE],
If [NTP DATE] is blank do nothing

Any help here would be greatly appreciated.