Hmm...I still don't understand what you want?

This code is what I believe you're asking for:

select 21 / (datediff(dd,getdate(),getdate()) + 1)

But you could just as easily replace (datediff(dd,getdate(),getdate()) + 1) by just 1.

select 21 / 1

Above code is the same.

Why do you want to subtract today from today ( GETDATE() - GETDATE() )? What's the purpose of doing that?