DateTime型についてのまとめ

日付型の宣言と計算方法

flutter pub add age_calculator

  DateTime birthday = DateTime.parse('1968-06-10');
  DateTime selecteddate1 = DateTime.now();
  DateTime selecteddate2 = DateTime.now().add(Duration(days: 30));
......

    final age = AgeCalculator.age(birthday, today: selecteddate1);
    final DateTime days14 = selecteddate1.add(Duration(days: 14));
    final DateTime days90 = selecteddate1.add(Duration(days: 90));
    final int daysselect = selecteddate2.difference(selecteddate1).inDays;
......

年齢計算その他色々

import 'package:age_calculator/age_calculator.dart';

void main() {
  DateTime birthday = DateTime(1997, 3, 5);

  DateDuration duration;

  // 今日が 2021-03-08　の場合。。
  duration = AgeCalculator.age(birthday);
  print('Your age is $duration'); // Your age is Years: 24, Months: 0, Days: 3

  final year = duration.years;
  final month = duration.months;
  final day = duration.days;
  return '$year歳$monthヶ月$day日'; // 24歳0ヶ月3日

  //Find out your age on any given date
  duration = AgeCalculator.age(birthday, today: DateTime(2030, 5, 1));
  print('Your age is $duration'); // Your age is Years: 33, Months: 1, Days: 26

  // Find out when your next birthday will be at 2021-03-08
  duration = AgeCalculator.timeToNextBirthday(birthday);
  print('You next birthday will be in $duration');
  // You next birthday will be in Years: 0, Months: 11, Days: 25

  // Find out when your next birthday will be on any given date
  duration = AgeCalculator.timeToNextBirthday(birthday,
      fromDate: DateTime(2021, 3, 2));
  print('You next birthday will be in $duration');
  // You next birthday will be in Years: 0, Months: 0, Days: 3

  // Find out the difference between two dates
  duration = AgeCalculator.dateDifference(
    fromDate: DateTime(2021, 1, 2),
    toDate: DateTime(2025, 5, 2),
  );
  print('The difference is $duration');
  // You next birthday will be in Years: 4, Months: 4, Days: 0

  // Add time to any date
  DateTime date = AgeCalculator.add(
      date: DateTime(2021, 1, 2),
      duration: DateDuration(years: 5, months: 2, days: 1));
  print(date);
  // 2026-03-03 00:00:00.000
}

DateTime型を日本語表記する関数

flutter pub add intl

import 'package:intl/intl.dart';
import 'package:intl/date_symbol_data_local.dart';  
.....

  String japanDate(DateTime dt) {
    const Locale("ja");
    initializeDateFormatting("ja");
    return DateFormat.yMMMd('ja').format(dt).toString() +
        '(${DateFormat.E('ja').format(dt)})';
  }

カレンダーから日付を選択するWidget

      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        textTheme: TextTheme(
          titleMedium: TextStyle(fontSize: 20),
          bodyMedium: TextStyle(fontSize: 18),
        ),
      ),
......

DateTime birthday = DateTime.parse('1968-06-10');
......

 Container(
   child: Row(
     mainAxisAlignment: MainAxisAlignment.center,
     children: <Widget>[
        Text(
          '生年月日:' + japanDate(birthday),
          style: Theme.of(context).textTheme.bodyMedium,
        ),
　       ElevatedButton(
          onPressed: () => _selectBirthDate(context),
          child: const Text('選択'),
        ),
     ],
   ),
 ),  
........

  Future<void> _selectBirthDate(BuildContext context) async {
    final DateTime? picked = await showDatePicker(
      context: context,
      initialDate: birthday,
      firstDate: DateTime(DateTime.now().year - 100),
      lastDate: DateTime(DateTime.now().year + 100),
    );

和暦表示（未整理　年のみで判断している）

  String convertYear(int year) {
   if(year < 1868) {
     return '明治より前です';
   } else if(year < 1912) {
     return '明治${year - 33 - 1900}年';
   } else if(year < 1926) {
     return '大正${year - 11 -1900}年';
   } else if(year < 1989) {
     return '昭和${year - 25 -1900}年';
   } else if(year < 2019) {
     return '平成${year + 12 -2000}年';
   } else {
     return '令和${year - 18 -2000}年';
   }
 }