Calculating difference between dates

Hi,
I want to calculate the number of days between two dates, on a spreadsheet where some of the dates are missing.

so, for example, I am trying to work our how long we have had a number of projects ongoing for. Some projects have already completed, so I have a closure date, but some projects are still ongoing, so I want to use today's date (today function).

In this Scenario:
Column A has the project start date
Column B has the closure date (with some cells blank where the project is still ongoing)
Column C has today's date (using the today() function).
Column D is where i want to work out the total number of days a project has been ongoing for.

Is there a single formula I can use in column D which will populate the total number of days either by calculating the difference between column B minus column A or, where column B is blank, Column C minus column A?

(NOTE: the data is in a table, so I cannot have two different formulae in column D by selecting the blank cells of column B only)