1. Close Numbers
2. Delete the Numbers preference file: home > Library > Preferences,
com.apple.iWork.Numbers.plist.
3. Relaunch Numbers it will create a clean com.apple.iWork.Numbers.plist.
4. Highlight the column, Click the B at the column top.
5. From Cell Inspector set the date format to what you desire
6. In column B click a cell then input a date. How does it perform?.

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bombholio