Hello & Welcome to the Board,
You'll need a worksheet change event macro for this.
You are right about the formula. The Now() function gives you the current date/time...so, every time you either close and reopen the workbook or hit F9 to recalcualte the worksheet the Now() function refires giving you the current data/time.
This is a worksheet module so it needs to be placed in the worksheet code. Right click on the sheet and select view code. Paste the macro above on the right side of the screen. Alt + Q will close the VBE and return you to Excel.
Go to the sheet and enter something in E. The cell just to the right in F will now get the date which is a hardcoded so will not change when you open the workbook or hit F9.
Here is some additional reading about where to place code.
Where to paste code
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