I create this formula and copied it down.

=DATE(YEAR(B5), MONTH(B5)+1, DAY(B5))

I have custom formatted the date: mm/yy

It works fine for most month sequences but when I enter an end of month date
like 31/01/2005 and copy it down....February is missing...

Mar-05
Apr-05
May-05

Can any one explain what I am overlooking?

Thanks in advance