onEdit Function for Google Sheets

GOOGLE APP SCRIPT

var sheetName = "Sheet1"

function onEdit() {

  var ss = SpreadsheetApp.getActiveSpreadsheet().getSheetByName(sheetName);

  var activeCell = ss.getActiveCell();

  var row = activeCell.getRow();

  var col = activeCell.getColumn();

  var timeStamp = ss.getRange(row,col-1).getValue();

  var rowHeader = ss.getRange(3,col).getValue();

  if(!activeCell.isBlank() && timeStamp == "" && rowHeader == "COMPLETED" && row > 3){

    ss.getRange(row, col-1).setValue(new Date());

  }

}

Use This OnEdit Function For Time Stamp