ElapsedTimer.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Timers;

namespace ElapsedTimer
{
    public class ElapsedTimer
    {
        public delegate void ElapsedTimeEventHandler(object sender, ElapsedTimeEventArgs e);
        public event ElapsedTimeEventHandler ElapsedTimeEvent;

        private Timer timer;
        private Stopwatch sw;

        /// <summary>
        /// タイマー開始 (ミリ秒でイベント発行するインターバルを設定してください)
        /// </summary>
        /// <param name="intervalMillisecounds">イベント発行するインターバル(ミリ秒)</param>
        public void Start(double intervalMillisecounds)
        {
            timer = new System.Timers.Timer();
            timer.Interval = intervalMillisecounds;
            timer.Elapsed += OnElapsedEventHandler;
            timer.Start();
            sw = new Stopwatch();
            sw.Start();
        }

        public void Stop()
        {
            timer?.Stop();
            sw?.Stop();
        }

        public void Restart()
        {
            timer?.Stop();
            timer?.Start();
            sw?.Restart();
        }

        private void OnElapsedEventHandler(object sender, System.Timers.ElapsedEventArgs e)
        {
            ElapsedTimeEvent.Invoke(this, new ElapsedTimeEventArgs(e.SignalTime, this.sw.Elapsed));
        }

    }

    public class ElapsedTimeEventArgs : EventArgs
    {
        /// <summary>
        /// イベント発行時の時刻
        /// </summary>
        public DateTime SignalTime { get; }

        /// <summary>
        /// 経過時間
        /// </summary>
        public TimeSpan ElapsedTime { get; }

        /// <summary>
        /// コンストラクタ
        /// </summary>
        /// <param name="signal"></param>
        /// <param name="elapsed"></param>
        public ElapsedTimeEventArgs(DateTime signal, TimeSpan elapsed)
        {
            SignalTime = signal;
            ElapsedTime = elapsed;
        }
    }
}

Program.cs

using System;

namespace ElapsedTimer
{
    class Program
    {
        static void Main(string[] args)
        {
            var et = new ElapsedTimer();
            et.ElapsedTimeEvent += OnElapsedTimeEvent;
            double interval = 200;
            et.Start(interval);

            while(true)
            {
                Console.WriteLine($"{interval}ms 毎にイベントが発行されます。なにかキーを押すとタイマーをリスタートします");
                Console.ReadKey();

                Console.WriteLine(Environment.NewLine);
                et.Restart();
            }
        }

        static void OnElapsedTimeEvent(object sender, ElapsedTimeEventArgs e)
        {
            string elapsedTime = $"{e.ElapsedTime.Hours:00}:{e.ElapsedTime.Minutes:00}:{e.ElapsedTime.Seconds:00}.{e.ElapsedTime.Milliseconds / 10:00}";

            Console.CursorVisible = false;
            Console.Write($"イベント発行時刻 {e.SignalTime:hh:mm:ss.ff} 経過時間 {elapsedTime}");
            Console.SetCursorPosition(0, Console.CursorTop);
        }
    }
}