reverse datepicker in Flutter

history_page.dart

import 'package:flutter/material.dart';
import 'package:date_picker_timeline/date_picker_timeline.dart';
import 'package:route_map/constants.dart';

class HistoryPage extends StatefulWidget {
  @override
  _HistoryPageState createState() => _HistoryPageState();
}

class _HistoryPageState extends State<HistoryPage> {
  final startDate = DateTime(2020, 07, 14);
  DateTime _selectedValue = DateTime.now();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("History"),
        ),
        body: Container(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              Container(
                child: DatePicker(
                  DateTime.now()
                      .subtract(Duration(days: _calculateDays(startDate))),
                  daysCount: _calculateDays(startDate) + 1,
                  width: 60,
                  height: 80,
                  initialSelectedDate: DateTime.now(),
                  selectionColor: kPrimaryColor,
                  selectedTextColor: Colors.white,
                  onDateChange: (date) {
                    // New date selected
                    setState(() {
                      _selectedValue = date;
                    });
                  },
                ),
              ),
            ],
          ),
        ));
  }

  int _calculateDays(var startDate) {
    final today = DateTime.now();
    final difference = today.difference(startDate).inDays;
    print(difference);
    return difference;
  }