Excel not recognising all dates

Occasionally, especially if your data includes dates moving from American (mm-dd-yyyy) to European (dd-mm-yyyy), you will see that Excel ‘recognises’ only some of the dates and not the others. You will know you have this issue if you see that all the dates that are considered valid by Excel are day 12 or less and the ones treated as text are day 13 or above. This is a serious issue and it means that not only the unrecognised dates are wrong but also the dates that you thought were recognised are incorrect.

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