As andy holaday says, using cell formatting to change the format does not actually change existing numbers (dates) to text. You can do this with text-to-column functionality:
Select the date column> Data> Text to Columns> Next> Next> in step 3 under "Column Data Format" select "text"> "Finish"
This converts existing dates, if you have input dates that you want to enter as text, you can add an apostrophe, as Polly says, or you can simply pre-format the input column as a text format.
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