Conditional formatting for blank cell based on another cells date

Hi,

Like many others before me, apologies if this has been answered, but i've searched high and low on the internet, tried the varied suggestions and for some reason, still doesn't work for me.

My business sends out plans to clients, for which we require their sign off. We chase them up when we remember, but it's getting to a stage that we need something visual to show us exactly what is outstanding, and what is fine.

The basis is that we enter a date into a cell that a plan has been sent to someone. The next column is blank, but is where we would type in what date the signed plan was received.

I require a formula (or multiple), that would highlight blank cells that are exceeding 15 days since their corresponding date left of them.

Please see attached spreadsheet with basic example of what i need.

Cheers
Trent