仅在第一次填充单元格时加上时间戳（如果再次更改该单元格，则不会更新）

Question

以下脚本使我每次在A栏上填充一个单元格时都能获得C栏的时间戳。

如果再次更改列中的单元格，则很遗憾时间戳会更新。我只想要第一次填充该单元格的时间戳。

我应该如何进行？

`function onEdit(event)
{ 
  var timezone = "GMT+1";
  var timestamp_format = "MM-dd-yyyy HH:mm:ss"; // Timestamp Format. 
  var updateColName = "A";
  var timeStampColName = "C";
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Feuille_1'); //Name of the sheet where you want to run this script.
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Feuille_2'); //Name of the sheet where you want to run this script.
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Feuille_3'); //Name of the sheet where you want to run this script.
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Feuille_4'); //Name of the sheet where you want to run this script.

  var actRng = event.source.getActiveRange();
  var editColumn = actRng.getColumn();
  var index = actRng.getRowIndex();
  var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues();
  var dateCol = headers[0].indexOf(timeStampColName);
  var updateCol = headers[0].indexOf(updateColName); updateCol = updateCol+1;
  if (dateCol > -1 && index > 1 && editColumn == updateCol) { // only timestamp if 'Last Updated' header exists, but not in the header row itself!
    var cell = sheet.getRange(index, dateCol + 1);
    var date = Utilities.formatDate(new Date(), timezone, timestamp_format);
    cell.setValue(date);
   }
 }