Can't Restart Count Down Clock

[elynch303]

Trying to use a restart button to restart the clock

 var startTime;
    void _restartTimer() async{
      startTime = DateTime.now();
      setState(() {});
    }
    return Material(
        child: Column(
          children: <Widget>[
            Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Container(
                    padding: const EdgeInsets.all(8.0),
                    margin: const EdgeInsets.symmetric(horizontal: 8.0),
                    child: FlipClock.countdown(
                      startTime: startTime,
                      duration: Duration(minutes: model.durationMin, seconds: model.durationSec),
                      digitColor: Colors.white,
                      backgroundColor: Colors.black,
                      digitSize: 48.0,
                      borderRadius: const BorderRadius.all(Radius.circular(3.0)),
                      onDone: () => print('Done'),
                    ),
                  ),
                  IconButton(onPressed: _restartTimer, icon: Icon(Icons.refresh))
                ]
            )
     ......