Do you understand relative vs absolute references? In your formula,
the entry for the Access date cell must remain absolute across the
row. The entry for the column name (i.e. 1-jan-05) must be absolute
copying down each column.

If you are creating a formula in the first cell, then simply copying
to other cells, your answers will be off unless you set the proper
relative and absolute references. Just hit F2 in any cell to see what
cells that formula is pointing to and you will see what I mean.