Hi,
I've not found a way to add a dynamic(scope) value in attr countdown.
Ex: or (This way throws error in parsing)
To fix this I changed the module directive adding the last line:
(countdown: '=', and it works)

  scope: {
    interval: '=interval',
    startTimeAttr: '=startTime',
    endTimeAttr: '=endTime',
    countdownattr: '=countdown',
    countdown: '=',

I'm missing something? There is another way?
Thanks.