Insert a helper column to the right of your errant dates and enter the
following formula in the first blank cell of the new column (substituting A1
with the cell containing the text date):
=DATEVALUE(A1)
Copy the formula down as far as necessary and then copy your formula column.
Move the the first cell of the original date column, click EDIT in the menu
and select PASTE SPECIAL and select the VALUES option button & click OK. You
can then delete your helper column.
You might have to format the column for dates, as it converts the text date
to a serial date number.