That should be...

1/1/05 (January 1, 2005)

2/1/05 (February 1, 2005)

3/1/05 (March 1, 2005)

....and so on.

In article ,
Domenic wrote:

The first row, starting at B1, contains the month entered in the
following manner...

1/1/2005

1/2/2005

1/3/2005

...and custom formatted as "mmmm"