lastmodifieddate on Support Case not working ...

I'm currently trying to represent the time a case has worked in number of minutes - but lastmessagedate and lastmodifieddate always seems to come up null.  Here's my code:

function updateMinutesWorked()
{

var recordType = nlapiGetRecordType();
var recordId = nlapiGetRecordID();
var record = nlapiLoadRecord( recordType, recordId );

var caseStatus = record.getFieldValue('status');
var body = recordType + ' ' + recordId + ' ' +  caseStatus;

if (caseStatus == 5)
{
 
  var startdate = record.getFieldValue('startdate');
  var starttime = record.getFieldValue('starttime');
 
  var caseStart = startdate + ' ' + starttime;

  var enddate = record.getFieldValue('lastmodifieddate');
  // var endtime = record.getFieldValue('')
 
  var minutesWorked = dateDiff('n', enddate, caseStart); 
  var newbody = record.getFieldValue('lastmodifieddate') + ' - ' + caseStart + ' = ' + minutesWorked + "\n" + record;