使用date1.difference(date2).inDays 不正确。在某些日期，它将返回错误的结果，例如DateTime(2020,3)。 这是因为持续时间是基于秒计算的。而且inDays的实现是在假设天总是具有相同秒数（_duration ~/ Duration.microsecondsPerDay）的前提下进行的。

但是，您可以使用

DateTime(date.year,date.month + 1,0).day

您可以简单地使用Quiver / Time库

import 'package:quiver/time.dart';

daysInMonth(year,month);

您可以使用软件包https://pub.dev/packages/date_util
使用dateUtility.daysInMonth

代码段

var dateUtility = DateUtil();
var day1 = dateUtility.daysInMonth(2,2019);
print(day1);
var day2 = dateUtility.daysInMonth(2,2018);
print(day2);

完整代码

import 'package:flutter/material.dart';
import 'package:date_util/date_util.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then,without quitting the app,try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",// or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,),home: MyHomePage(title: 'Flutter Demo Home Page'),);
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key,this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful,meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    var dateUtility = DateUtil();
    var day1 = dateUtility.daysInMonth(2,2019);
    print(day1);
    var day2 = dateUtility.daysInMonth(2,2018);
    print(day2);

    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State,which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(),then the build method would not be
      // called again,and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called,for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast,so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method,and use it to set our appbar title.
        title: Text(widget.title),body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default,it sizes itself to fit its
          // children horizontally,and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console,choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio,or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,children: <Widget>[
            Text(
              'You have pushed the button this many times:',Text(
              '$_counter',style: Theme.of(context).textTheme.display1,],floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,tooltip: 'Increment',child: Icon(Icons.add),// This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

输出

I/flutter (11462): 28
I/flutter (11462): 28

无需使用任何软件包，您可以使用以下软件包：

DateTime x1 = DateTime(2019,1,0).toUtc();
var y1 = DateTime(2019,2,0).toUtc().difference(x1).inDays;

这应该做到。

void main() {
  var now = DateTime.now();
  print(daysInMonth(now));
}

int daysInMonth(DateTime date){
  var firstDayThisMonth = new DateTime(date.year,date.month,date.day);
  var firstDayNextMonth = new DateTime(firstDayThisMonth.year,firstDayThisMonth.month + 1,firstDayThisMonth.day);
  return firstDayNextMonth.difference(firstDayThisMonth).inDays;
}

打印：

30

您可以简单地计算天数，而无需任何第三方库。

int daysInMonth = DateTimeRange(
                       start: date,end: DateTime(date.year,date.month + 1))
                    .duration
                    .inDays;

int year = DateTime.now().year;

int month = DateTime.now().month;

DateTime thisMonth = DateTime(year,month,0);

DateTime nextMonth = DateTime(year,month+1,0);

int days =  nextMonth.difference(thisMonth).inDays;

print("Days In This Month: $days");