Excel recognizes a number as a date by its date format, which uses a slash (/) as a separator, for example, 1/25/2004 is recognized as a date by Excel.
To change the default setting for the date format:
1. From the Windows Start menu, select Settings, Control Panel, and then Regional and Language Options.
2. In Regional Options, select Customize.
3. Select the Date tab, and in the Date separator dropdown list, change the slash (/) to a dot (.).
4. Click Apply and then click OK.
Screenshot // Changing the Default Date Separator from a Slash to a Dot When Entering a Date into a Cell