I don't think it is possible to assign a value to a number, you can only
assign a value to a cell. Also, your example refers to 2 cells, B3 and B5, is
it a typo?

However, why not use some available features of Excel, like using named
ranges?

Go to cell B3, then on the formula bar, on the left where B3 is written,
click it and put for example AddDays. Now, in your cell where you would like
to use the content of this cell, simply add at the end of the formula
+AddDays.

If this is not what you want, let us know!

Stephane.