You can easily convert columns in Excel so that they can be formatted as dates by using the Text to Columns wizard. While in your Excel table, follow these steps: 
  1. First, use the column header to select the entire column with data to convert
  2. In the menu ribbon, select the Data tab
  3. Within the Data Tools section, click the Text to Columns button 
  4. Follow these steps within the resulting wizard pop up
    1. Step 1 – no changes
    2. Step 2 – no changes
    3. Step 3 – Select Date and then if needed also pick the appropriate layout order (MDY = month day year, ex. 123187 for Dec 31 1987). It just has to match up the order that the data lives in the Excel column
  5. Once you’ve completed the wizard, dates will appear as the 12/31/1987 style and Excel will have assigned them the “date format” as well.