If I'm not mistaken, it is a formatting issue. Not the font, but the number format. From the Home tab on your ribbon, press the little boxy thing with the arrow on the bottom right of the Number section to open up the Format Cells dialog box.

In the Category list box, select Date if it is not already selected. Then scroll through the available formats in the Type list box. Otherwise, choose custom in the Category list box and make your own.