Timer is skipping minutes and not working correctly after few minutes

I have created a timer that starts from 30 minutes and goes to 0. But I noticed that it only works correctly for the first 3 minutes but after that, it starts skipping minutes and sometimes stops or goes back to 0 instantly. To showcase the issue I have created a dartpad which you can run here - https://dartpad.dev/0f8bb5a5cf41f550f406c8fd7c93f3a6.

The same full code is this:-

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

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

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

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Timer _timer;
  int _start = 30;

  void startMinTimer() {
    const oneMin = const Duration(minutes: 1);
    _timer = new Timer.periodic(
      oneMin,
      (Timer timer) {
        if (_start == 0) {
          setState(() {
            timer.cancel();
          });
        } else {
          setState(() {
            _start--;
          });
        }
      },
    );
  }

  @override
  void dispose() {
    _timer.cancel();
    super.dispose();
  }

  //Todo: fix timer automatically resets to 0 after sometime
  @override
  Widget build(BuildContext context) {
    startMinTimer();
    return Scaffold(
      appBar: AppBar(
        title: Text('Timer Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'Minutes Left',
            ),
            Text(
              '$_start',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
    );
  }
}

I want to knows what wrong is happening and how I could fix that. Thanks in advance.