|You need to Custom Format the cell or cells.|
Select the cell or range of cells you want to re-format.
Select the Home Tab
Select Format Cells (Bottom of list)
In the Pop Up window:
Select Number Tab
In the Type input box enter: MM-DD-YYYY
See how that works for you.
Sorry, I misread your post
DerbyDad03 is correct, you cannot enter just the numbers you will need to also use the Dash and the format will be 01-01-2015.
The only other way is with a Macro.
The Dash or Slash tells Excel that it is a Date not just another number.
message edited by mmcconaghy