I have dates from two different Excel files in one Excel file. In one file, the dates were entered with the 1904 date system checked. In the other one, it was not. Now that both sets of data are in one excel file, if I uncheck the 1904 date system, one set of dates shows 2010 instead of 2014. If I keep it checked, one set of dates adds one day and four years (e.g., 12/4/2014 becomes 12/5/2018).

How can I compare these two dates?