To summarize:

To combine the month, day, and year into one cell:
To change the format of the date:
1. Select the cell with the date you want to format.
2. Right-click and select Format Cells.
3. In the Format Cells dialog box, select the Number tab.
4. In the Category list, select Date.
5. In the Type list, select the format you want.
6. Click OK.

To calculate the number of days between the specified date and a start date: