Google Spreadsheet Timestamp for columns B & D?

Google Spreadsheet Timestamp for columns B & D?

Found this script on one of the questions answered in the forum what I don't know enough to make it work. I would like it to time-stamp date and time in column B when they check in and then a separate script when the clock out to time-stamp date and time in column D. Not sure how to do this google spreadsheet. Here is the script below that I found but it will only time-stamp column N when changes are made in column M. Let me know how to change this.
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() == 13 ) { //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);
   };
 };
}