Number Formats For Date

I have a simple piece of code that gets a date from the user via inputbox in the format dd/mm/yyyy. That date is then put into a cell (e.g. B1), and the cells to its right get given something like "=B1+2", "=C1+5" etc so that they will all show a date a few days further down the track. (Much more complicated than that but this is the basic idea)

The entire row is then formatted "ddd dd mmm yyyy" with the intention of every column getting a header like "Wed 27 Jan 2010". This works perfectly for every column EXCEPT B which shows the the string "27/01/2010" it was given from the inputbox.

Right clicking --> Format Cells shows every cell in the row has the exact same 'Custom' format, and the 'Sample' at the top looks correct for every cell except B1.

BEFORE the macro runs and formats all the cells, but AFTER they have been given their formulas, they all display something along the lines of '40021', '40023' which I assume is the number of days since Jan 01 1900 or the like - proving that THEY knew the value in B1 was a date, so why doesnt B1 itself know?