There are two dates fromat in MS Excel. One comes from an old version of Excel running on Macintosh Computer. The second one is the standard 1900 format, which is being used now. If you came accross the problem with converting the 1904 and 1900 dates, we bring you a simple solution.

1. Open the original workbook and go to Tools > Options > Calculation to check if you have enabled 1904 Date System option. If so, you are using a file taken from an old MAC OS Excel version.
2. Open a new blank workbook with 1904 option disabled.
3. Paste all data from the original workbook to the new blank one. You can see all dates have changed in new workbook.
4. In an empty cell, enter the value 1462. That is the difference between two date types.
5. Select the cell and copy it into the clipboard (ctrl + c)
6. Select the cells that contain the shifted dates. On the Edit menu, click Paste Special > Paste as Values.
7. Then, select either of the following option buttons.  
   Add  -The dates must be shifted up by four years and one day. To convert date from 1904 to PC 1900
   Subtract -The dates must be shifted down by four years and one day.
8. Click OK. Repeat those steps until all of the shifted dates have been corrected.