How to switch between two cells using VBA?

As you can see in the attached file, I have two different dates: one is set manual and one is automatic using "today" function. There is another cell called "project start date" which is linked to B1 which is a manual date. from time to time, I change "project start date" from one to another. Now, I want to be able to shift the content of project start date" from manual to automatic or vice versa using a button (which I suppose would be possible using "insert control").