excel displays number for date

In my excel spreadsheet Column D, I have dates that display as numbers in the cell but in the formula box show as a date. These have not been imported, I manually entered all of them. Something I did and don't aske me what ahs changed the formatting. Some of the cells in Column D show as a date but I have no idea why. I have tried formatting the cells to date but nothing is working.  What am I missing?