Windows service run for every one hour

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C#3.5

I have a windows service which should run for every one hour. But the issue is Ex: it starts at 6.00 A.m and ends at 6.03 A.M. Then again after one hour its starting running at 7.03 A.M. But I want it to run again at 7.00 A.M.
Anyone's help will be appreciated.
The code What I have written is below

What I have tried:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Linq;
using System.ServiceProcess;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace Ashworth.WindowService.GradeDataFeed
{
    public partial class GradeDataFeedService : ServiceBase
    {
            private readonly System.Timers.Timer _timer;
            GradeDataFeedManager gradeDataFeedManager;
            //Thread managerThread = null;

            public GradeDataFeedService()
            {
                InitializeComponent();

                _timer = new System.Timers.Timer();
                _timer.Elapsed += timer_Elapsed;
            }

            protected override void OnStart(string[] args)
            {
                _timer.Interval = 60 * 1000;
                _timer.Enabled = true;
                _timer.Start();
            }

            private void timer_Elapsed(object sender, EventArgs e)
            {
                // Only create / start the manager if has been done so already
                // It should only all in once, since the manager object will not
                // be null and since the timer will be disabled after first run
                if (gradeDataFeedManager == null)
                {
                gradeDataFeedManager = new GradeDataFeedManager();

                // Create thread for manager
                //managerThread = new Thread(shipmentTrackingNumberProcessorManager.Start);

                // Start the thread
                gradeDataFeedManager.Start();

                    // Stop timer, it only needs to run once to kick off the manager
                    _timer.Enabled = false;
                }
            }

            protected override void OnStop()
            {
                if (gradeDataFeedManager != null)
                {
                gradeDataFeedManager.Stop();
                }
            }
        }
    }