Flutter dynamically update date and time - list

Question

I am new to flutter and this is my first app. I am trying to make a to do list app and want to display the time left for each task in the subtitle. I have a listview and in each element I want the have the subtitle display the minute, counting downwards towards 0. Can anyone help me with this ? Thanks!
Code : -
class toDoListState extends State<toDoList>
{
List<String> tasks = [];
List<String> completedTasks = [];
List<String> descriptions = [];
List<bool> importance = [];
List<String> time2completion = [];
List<DateTime> time = [];
Widget buildToDoList()
{
return new ListView.builder
(
itemBuilder: (context, index)
{
if(time2completion.length > 0 && index < time2completion.length && time2completion[index] != "none")
{
if(time2completion[index] == "30 minutes")
{
time[index] = DateTime.now().add(Duration(minutes: 30));
}
else if(time2completion[index] == "1 hour")
{
time[index] = DateTime.now().add(Duration(hours: 1));
}
else if(time2completion[index] == "12 hours")
{
time[index] = DateTime.now().add(Duration(hours: 12));
}
else if(time2completion[index] == "1 day")
{
time[index] = DateTime.now().add(Duration(days: 1));
}
}
if(index < tasks.length)
{
return row(tasks[index], descriptions[index], index);
}
},
);
}
Widget row(String task, String description, int index)
{
return Dismissible(
key: UniqueKey(),
background: Container(color: Colors.red, child: Align(alignment: Alignment.center, child: Text('DELETE', textAlign: TextAlign.center, style: TextStyle(color: Colors.white, fontSize: 18),))),
direction: DismissDirection.horizontal,
onDismissed: (direction) {
setState(() {
tasks.removeAt(index);
if(completedTasks.contains(task))
{
completedTasks.removeAt(index);
}
descriptions.removeAt(index);
importance.removeAt(index);
});
Scaffold.of(context).showSnackBar(SnackBar(content: Text(task+" dismissed")));
},
child: CheckboxListTile(
controlAffinity: ListTileControlAffinity.leading,
title: Text(task, style: (completedTasks.contains(task)) ? TextStyle(decoration: TextDecoration.lineThrough) : TextStyle(),),
subtitle: Text((time[index].difference(DateTime.now()).toString())),
value: completedTasks.contains(task),
onChanged: (bool value) {
setState(() {
if(!completedTasks.contains(task))
{
completedTasks.add(task);
}
else
{
completedTasks.remove(task);
}
});
},
));
}
}

Answer 1

You can use a timer to calculate the time differences every minute.
Timer.periodic(
Duration(minutes: 1),
(Timer t) => setState(() {
// your calculation here
}),
);
The following will create a timer object in your stateful widget and dispose of it when you navigate away from the view:
Timer _timer;
#override
void initState() {
_timer = Timer.periodic(
Duration(minutes: 1),
(Timer t) => setState(() {
// your calculation here
}),
);
super.initState();
}
#override
void dispose() {
_timer.cancel();
super.dispose();
}

Answer 2

You can achieve this by following:
Suppose you already have saved date in your todo and have already a date available in
form of DateTime object here in my example I am assuming savedDateTime which can be achieved either by assigning it using .hour , .sec or parsing it from string.
Now what you do is to find what time is left that is differnce
//already assumed saved date as a DateTime object that is savedDateTime
// it contains our saved date of note
//now
final currentDateTime = DateTime.now();
final difference = currentDateTime.difference(savedDateTime);
difference in Seconds,Hours,Minutes,Days is given by
print(difference.inSeconds);
print(difference.inHours);
print(difference.inMinutes);
print(difference.inDays);