Code.gs

function onEdit() {
  var cell = SpreadsheetApp.getActiveSheet().getActiveCell();
  
  switch (cell.getColumn()) {
    case 2:
      insertTimestamp(cell, 'C');
      break;
    case 4:
      insertTimestamp(cell, 'E');
      break;
    case 7:
      insertTimestamp(cell, 'H');
      break;
    case 9:
      insertTimestamp(cell, 'J');
      break;
  }
}

function insertTimestamp(modifiedCell, timestampColumn) {
  var row = modifiedCell.getRow();
    
  var offset = "GMT-"+Math.floor((new Date()).getTimezoneOffset() / 60);
  var time = Utilities.formatDate(new Date(), offset, "hh:mm a");
    
  SpreadsheetApp.getActiveSheet().getRange(timestampColumn + row.toString()).setValue(time);
}