Countdown timer in streambuilder - Flutter

Hello everyone i'm a beginner on Flutter:), I want to create a countdown with a given time, but I can't update the value in my StreamBuilder.

this code is in my streambuilder :

 return Expanded(
                child: ListView(
              children: snapshot.data!.docs.map((DocumentSnapshot document) {
                Event event = Event.fromMap(
                    document.data() as Map<String, dynamic>, document.id);

                return Container(
                    color: Colors.green[400],
                    margin: const EdgeInsets.all(8),
                    child: Padding(
                        padding: const EdgeInsets.all(16),
                        child: Row(
                          children: [
                            Expanded(
                                child: Column(
                              crossAxisAlignment: CrossAxisAlignment.start,
                              children: [
                                Text(event.name),
                                const SizedBox(height: 16),
                                Text(FormatDateHelper()
                                    .dateFormated(event.date)),
                                const SizedBox(height: 16),
                                Text(event.street),
                              ],
                            )),

                            ....

Could someone help me please?