Excel 2000

I have an imported file with data in column AB. I enter a 'post date' in
column AC, row 1, but when I highlight the cell and double click on the box,
the date increases as it goes down the column. When I hold down the CTRL key
and double click on the box, it also increases as it goes down the column.
If I manually draw AC1 down with the CTRL key pressed, it works the way I
want - i.e. 02/29/08 is in each cell in the column. Is there a macro I
should use?

TIA,
CaroleO