Input date in cell based on another cell without updating the date

I've done this before in another application, so I don't know why this is so difficult right now. I have a spreadsheet, and when I put a value in a cell, I want today's date to pop in another cell, but I don't want the date to update to today when I re-open the spreadsheet at a later date.

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The only difference is, my autofill of the date is dependent upon the contents of another cell. I know I have to do VBA for this, but I don't know the function to use. I thought it would be like this:

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I guess I just don't know how to use the Target parameter. Plus, I need to avoid the infinite loop, which means I need code to only look for change in cell C4, rather than the whole sheet.

Other than the missing "Then", the code compiles. Nothing happens when I type something in cell C4 though.

That did it Thank you so much!