I am glad that you like and you asked a valid question.

• Why there is not 59 second?

Stream<int> _stream() {
     Duration interval = Duration(seconds: 1);
     Stream<int> stream = Stream<int>.periodic(interval, transform);
     stream = stream.take(59);
     return stream;
}
int transform(int value) {
     print(value);   // add this line
     return value;
}

Provides at most the first count (59 in our case) data events of this stream (Source)

It starts with 0 seconds and goes to 58 seconds, which should be i.e start with 1 second and goes till 59 seconds. So we need to transform the value

• This is how you can transform the value.

How to fix this issue?

int transform(int value) {
    return value + 1;
  }
}

Here is full source code

import 'package:flutter/material.dart';
void main() {
  runApp(MaterialApp(
    home: HomePage(),
    title: 'Stream Demo',
  ));
}
class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Stream Demo'),
      ),
      body: Center(
        child: StreamBuilder(
          builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
            if (snapshot.connectionState == ConnectionState.done) {
              return Text(
                'Done : 00:${(snapshot.data).toString().padLeft(2,'0')}', //updated
                style: TextStyle(
                  fontSize: 30.0,
                ),
              );
            } else if (snapshot.connectionState == ConnectionState.waiting) {
              return Text(
                'Waiting For Stream',
                style: TextStyle(
                  fontSize: 30.0,
                ),
              );
            }
            return Text(
              'Active: 00:${(snapshot.data).toString().padLeft(2,'0')}',  //updated
              style: TextStyle(
                fontSize: 30.0,
              ),
            );
          },
          initialData: 0,
          stream: _stream(),
        ),
      ),
    );
  }
Stream<int> _stream() {
    Duration interval = Duration(seconds: 1);
    Stream<int> stream = Stream<int>.periodic(interval, transform);
    stream = stream.take(59);
    return stream;
  }
int transform(int value) {
    return value + 1; // updated 
  }
}

One important thing is there which I totally forget to mention that the last data of stream comes in Done State