如何找到颤抖中的两个日期之间的差异？

我正在尝试一个flutter应用程序，该应用程序可以计算两个日期之间的差异并以天为单位显示。

但是由于某种原因，我无法理解正确的逻辑。我创建了足够的变量来保存当前日期，并创建了另一个变量来保存旧日期，但是仍然缺少一些东西。

我使用了FLutter_datetime_picker包。

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_datetime_picker/flutter_datetime_picker.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData.dark(),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    final currentDate = DateTime.now();
    final oldDate = DateTime.tryParse();
    final difference = currentDate.difference(oldDate).inDays;
    return Scaffold(
      body: Row(
        children: <Widget>[
          Center(
            child: RaisedButton(
                child: Text("data"),
                onPressed: () => DatePicker.showDatePicker(context,
                        showTitleActions: true,
                        currentTime: currentDate,
                        minTime: DateTime(1990, 1, 1), onConfirm: (date) {
                      setState(() {
                        date = oldDate;
                      });
                      print(difference);
                    })),
          ),
          Text(difference.toString())
        ],
      ),
    );
  }
}