I have a way to do that, no need to use macro or change the system clock,
but a little bit much operation
after you entered all the date of the year 2005 as default, you just enter
365 in a cell and copy it, then select all the cells with the date you want
to change to 2004, from menu Edit-pastespecial-substract
.. You may get a lot of numbers, change the cell format to date at last.