Confirmation of specified range

check_date.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});
  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  int check = 0;
  String tex = '';

  void _incrementCounter() {
    setState(() {
      _counter++;
      check = DateTime.now().checkDate(
        DateTime.now(),
        DateTime(
          DateTime.now().year,
          _counter,
          DateTime.now().day,
        ),
      );
      if (check.isNegative) {
        tex = 'before';
      } else {
        tex = 'after';
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              '${DateTime.now()}',
            ),
            Text(
              _counter.toString() + tex + check.toString(),
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

extension CheckDate on DateTime {
  int checkDate(
    DateTime date,
    DateTime checkDate,
  ) {
    return date.difference(checkDate).inDays;
  }
}

2022-10-11.23.56.12.mov