参考：
https://blog.csdn.net/huanongjingchao/article/details/38374233?locationNum=5&fps=1

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.wear.widget.BoxInsetLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="@dimen/box_inset_layout_padding"
    tools:context=".MainActivity"
    tools:deviceIds="wear">

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:padding="@dimen/inner_frame_layout_padding"
        app:layout_boxedEdges="all">
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical"
            >
            <LinearLayout
                android:id="@+id/top"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:layout_marginTop="20dp"
                android:orientation="horizontal" >

                <TextView
                    android:id="@+id/mint"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="00"
                    android:textSize="30dp" />

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text=":"
                    android:textSize="30dp" />

                <TextView
                    android:id="@+id/sec"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="00"
                    android:textSize="30dp" />
            </LinearLayout>

            <LinearLayout
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_below="@+id/top"
                android:layout_centerHorizontal="false"
                android:layout_marginTop="60dp"
                android:orientation="horizontal" >

                <Button
                    android:id="@+id/start"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="start" />

                <Button
                    android:id="@+id/reset"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="reset" />
            </LinearLayout>
        </LinearLayout>
    </FrameLayout>
</androidx.wear.widget.BoxInsetLayout>

mainActivity

package com.example.miaobiao;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

import com.example.miaobiao.databinding.ActivityMainBinding;

import java.lang.ref.WeakReference;

public class MainActivity extends Activity {
    private TextView mint;
    private TextView sec;
    private Button start;
    private Button reset;
    private long timeusedinsec;
    public boolean isstop = false;
    private Handler mHandler = new MyHandler(this);
    static class MyHandler extends Handler {
        private final WeakReference<Activity> mActivity;
        MyHandler(Activity activity){
            this.mActivity = new WeakReference<>(activity);
        }
        @Override
        public void handleMessage(Message msg) {
            MainActivity activity = (MainActivity) mActivity.get();
            if(activity != null)
            {
                super.handleMessage(msg);
                activity.handleMyMessage(msg);
            }
        }
    }
    public void handleMyMessage( Message msg )
    {
        switch (msg.what) {
            case 1:
                // 添加更新ui的代码
                if (!isstop) {
                    updateView();
                    mHandler.sendEmptyMessageDelayed(1, 1000);
                }
                break;
            case 0:
                break;
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initViews();
    }
    private void initViews() {
        mint = (TextView) findViewById(R.id.mint);
        sec = (TextView) findViewById(R.id.sec);
        reset = (Button) findViewById(R.id.reset);
        start = (Button) findViewById(R.id.start);
        reset.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub

                mint.setText("00");
                sec.setText("00");
                start.setText("start");
                timeusedinsec=0;
                isstop=true;
            }
        });
        start.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                mHandler.removeMessages(1);
                String aaa=start.getText().toString();
                if(aaa.equals("start")){
                    mHandler.sendEmptyMessage(1);
                    isstop = false;
                    start.setText("pause");
                }else {
                    mHandler.sendEmptyMessage(0);
                    isstop = true;
                    start.setText("start");
                }

            }
        });
    }
    private void updateView() {
        timeusedinsec += 1;
        int minute = (int) (timeusedinsec / 60)%60;
        int second = (int) (timeusedinsec % 60);
        if (minute < 10)
            mint.setText("0" + minute);
        else
            mint.setText("" + minute);
        if (second < 10)
            sec.setText("0" + second);
        else
            sec.setText("" + second);
    }
}

效果：