Populating cell with a date after another cell has been populated

I have a spreadsheet that I want to record the date and time that another particluar cell was populated.
I have used the following formula:

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If I go into the spreadsheet at a later time, the cell will be updated. I would like to keep the original time and date.
Is it possible to keep the old date and time, and not have it over written?
I was thinking that when I exit the spreadsheet I could have Code that pastes special - values.
Could someone please advise me of the best possible solution, if in fact, there is one?
Thank you for any help you can offer.