sample code on how to use flutter date picker

main.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.light(),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  DateTime selectedDate = DateTime.now();

  Future _selectDate(BuildContext context) async {
    final DateTime picked = await showDatePicker(
        context: context,
        initialDate: selectedDate,
        firstDate: DateTime(1700, 8),
        lastDate: selectedDate,
        builder: (BuildContext contact, Widget child) {
          return Theme(
            data: Theme.of(context).copyWith(
              primaryColorDark: Theme.of(context).scaffoldBackgroundColor,
              dialogBackgroundColor: Theme.of(context).scaffoldBackgroundColor,
              secondaryHeaderColor: Colors.green,
              primaryColor: Colors.green,
              accentColor: Colors.green,
            ),
            child: child,
          );
        });
    if (picked != null && picked != selectedDate)
      setState(() {
        selectedDate = picked;
        print(selectedDate);
      });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text('using date picker')),
        body: Column(
          children: [
            Text('date selected is: $selectedDate'),
            SizedBox(height: 30),
            FlatButton(
                color: Colors.red,
                onPressed: () {
                  _selectDate(context);
                },
                child: Text('select date'))
          ],
        ));
  }
}