Flutter:有没有办法在给定的时间开始秒表课程？

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

class StopwatchPage extends StatefulWidget {
  @override
  _StopwatchPageState createState() => _StopwatchPageState();
}

class _StopwatchPageState extends State<StopwatchPage> {
  Timer _timer;
  int _seconds = 0;

  void startStopwatch(DateTime startTime) {
    Duration duration = DateTime.now().difference(startTime);
    _seconds = duration.inSeconds;
    _timer = Timer.periodic(Duration(seconds: 1), (Timer timer) {
      setState(() {
        _seconds++;
      });
    });
  }

  void stopStopwatch() {
    if (_timer != null) {
      _timer.cancel();
      _timer = null;
    }
  }

  @override
  void dispose() {
    stopStopwatch();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Stopwatch'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              '$_seconds seconds',
              style: TextStyle(fontSize: 24),
            ),
            SizedBox(height: 20),
            RaisedButton(
              onPressed: () {
                DateTime startTime = DateTime.now().add(Duration(seconds: 5)); // 设置开始时间为当前时间5秒后
                startStopwatch(startTime);
              },
              child: Text('Start Stopwatch'),
            ),
            RaisedButton(
              onPressed: stopStopwatch,
              child: Text('Stop Stopwatch'),
            ),
          ],
        ),
      ),
    );
  }
}