Hello, please help me out with this one.

So I am at work workin on this report and I need a function that will make Excel automatically insert the today's date in a cell (let's say D90) when I fill the content of another cell (for example C90) and the that will be inserted will never change. I tried to use the =TODAY( ) function but it keeps updating the date in the cell to the current date if i reopen the worksheet 2 days later. I need the cell to keep in the cell the date of the day when I filled in the content of cell C90 and do not updated it every time i open the document to the actual date.. Thanks in advance!!