Stopwatch.html

<template>
    <div style="background: white;padding: 20px;border-radius: 6px;">
        <div style="font-size: 20px;text-align:center;">{timeVal}</div>

        <template if:true={showStartBtn}>
            <lightning-button label="Start" onclick={start} style="margin-left: 145px;"></lightning-button>
        </template>
        <template if:false={showStartBtn}>
            <lightning-button label="Stop" onclick={stop} style="margin-left: 145px;"></lightning-button>
        </template>
        
        <lightning-button label="Reset" onclick={reset} style="margin-left: 10px;"></lightning-button>
    </div>
</template>

Stopwatch.js

import { LightningElement, track } from 'lwc';

export default class Stopwatch extends LightningElement {
    @track showStartBtn = true;
    @track timeVal = '0:0:0:0';
    timeIntervalInstance;
    totalMilliseconds = 0;

    start(event) {
        this.showStartBtn = false;
        var parentThis = this;

        // Run timer code in every 100 milliseconds
        this.timeIntervalInstance = setInterval(function() {

            // Time calculations for hours, minutes, seconds and milliseconds
            var hours = Math.floor((parentThis.totalMilliseconds % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
            var minutes = Math.floor((parentThis.totalMilliseconds % (1000 * 60 * 60)) / (1000 * 60));
            var seconds = Math.floor((parentThis.totalMilliseconds % (1000 * 60)) / 1000);
            var milliseconds = Math.floor((parentThis.totalMilliseconds % (1000)));
            
            // Output the result in the timeVal variable
            parentThis.timeVal = hours + ":" + minutes + ":" + seconds + ":" + milliseconds;   
            
            parentThis.totalMilliseconds += 100;
        }, 100);
    }

    stop(event) {
        this.showStartBtn = true;
        clearInterval(this.timeIntervalInstance);
    }

    reset(event) {
        this.showStartBtn = true;
        this.timeVal = '0:0:0:0';
        this.totalMilliseconds = 0;
        clearInterval(this.timeIntervalInstance);
    }
}

Stopwatch.js-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="Stopwatch">
    <apiVersion>45.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__AppPage</target>
        <target>lightning__RecordPage</target>
        <target>lightning__HomePage</target>
    </targets>
</LightningComponentBundle>