To convert a serial number to a date format, follow the steps below. Select the cell you want to format. On the Home tab in the Number group, click the bottom right arrow.

When this happens, use the DATEVALUE function to convert the text dates to Excel serial numbers. To test this function, enter five dates—as text—in cells A5 through A9.

Dates and times are stored as serial numbers in Microsoft Excel, ... To fix this, you'll need to convert all the data to time-date formats. See example 7 for more details.

These serial numbers constitute Excel's internal storage format for date entries. For example, "2012" entered into a date-formatted cell becomes July 5, 1909 in Excel for the Mac and July 4, 1905 ...