How to make a cell that saves the modification date of a specific range of cells?

I'm trying to create a cell that automatically saves the modification date of a specific range of cells. I don't know the best way to do it, but I think that with a script it would be something like this:

function onEdit(e){
 var DateCol = "K8";
 var curDate = Utilities.formatDate(new Date(), "UTC+1", "hh:mm")
 var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheets()[0];
 var range = sheet.getRange('C11:K18');
 //range.activate();
 var rowIndex = range.getRowIndex();
 var colIndex = range.getColumnIndex();

 // May need to set up multiple ranges like this and check all ranges if
 // checked fields are not in adjacent cells
 var watchRange = { // B2:G20
   top : 11,         // start row
   bottom : 18,     // end row
   left : 3,        // start col
   right : 11,        // end col
 };
 // Exit if we're out of range
 if (rowIndex < watchRange.top || rowIndex > watchRange.bottom) return;
 if (colIndex < watchRange.left || colIndex > watchRange.right) return;

 var currentValue = range.getValue();
 currentValue = currentValue.replace(/^\s+|\s+$/g,""); //"trim" kludge

 if (currentValue.length == 0){

 // Set a column to show last edit date
 SpreadsheetApp.getActiveSheet().getRange(DateCol + rowIndex).setValue("Edited: " + 
 curDate);

 }
}

Right now it does not give me an execution error, but it does not write the date in the cell and it selects the range of all the sheets and not one in particular.

I have managed not to select all the sheets by modifying line 4

var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Test2');

I have modified line 26

if (currentValue.length != 0)

Modifying line 29 writes me in the cell that I want

SpreadsheetApp.getActiveSheet().getRange(DateCol).setValue(curDate);

The only thing that fails me is that it puts me 1 hour less whatever I do.

I have modified this and it has been fixed

var curDate = Utilities.formatDate(new Date(), "GMT+01:00", "hh:mm");