Using Data Text to columns will work if the data is still in text format.
However, if Excel converted the date and time stamps to dateserial values
(numbers), then you may have to insert a new column to the right of the
Date/Time, and use a formula:

B1: =Int(A1)

Copy and PasteSpecial the values back into the column, then delete the
original column of Date/Time.

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Regards,
Bill Renaud