Google Sheets Date/Time Stamp

Question

I have a Google Sheet with a dropdown menu in column U.I would like column V to timestamp when column U is changed. I've used the following code however this only timestamps the first time that column U is changed, not any subsequent time when the drop-down is changed.

Any suggestions for how to make this work?

function onEdit() {
 var s = SpreadsheetApp.getActiveSheet();
 if( s.getName() == "Sheet1" ) { //checks that we're on the correct sheet
   var r = s.getActiveCell();
   if( r.getColumn() == 21 ) { //checks the column
     var nextCell = r.offset(0, 1);
     if( nextCell.getValue() === '' ) //is empty?
       var time = new Date();
       time = Utilities.formatDate(time, "GMT", "HH:mm:ss");
       nextCell.setValue(time);
   };
 };
}

Answer 1

Remove the if( nextCell.getValue() === '' ) bit.

The function is currently probably throwing an error when nextCell contains a value because of missing block delimiters {}, which cause time to be undefined. That will get fixed by the above change as well.