Auto insert a Static date

I was wondering if there is a way to automatically insert the static date in a worksheet if value is entered in another cell.

Example/reason.

I have a UAT testing log, and when the customer selects ‘Complete’ from a dropdown, (in cell I4), then I want the static date to be populated into another cell (O4) to show and record the date the action was completed.

Any advice most welcome.

Much Obliged