Created Date Problem

I have a form set up. I used this to try and replace an older process in my organization and its working great. One of my columns auto generates the created date, which is what I want. My problem comes when trying to upload historical data into the sheet. I have to change the column properties to text/numerical to be able to paste the date in my historical data. I need historical data to retain the original date. When I switch the column back to Created Date, its reverts all my newly uploaded data to todays date.

Sorry if this isn't very clear.

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