Know when the computer was turned On / Off / Stopped

The problem mentioned in the question regarding the date presented is because you were receiving a list and were not querying the value for an element of it, just the default of its structure.

I would use a Helper to make it easier to map and query the events you want, below is a basic template (using your own code) and an implementation example. This way you will have complete freedom to work with EventLogEntry in the most useful way.

QueryEventsHelper

public static class ConsultaEventosHelper
{
    private const long inicializacao = 2147489653;
    private const long desligamento = 2147484722;
    private const long reinicializacao = 2147489654;

    public static List<EventLogEntry> Inicializacao { get { return ConsultaOcorrencias(inicializacao); } }
    public static List<EventLogEntry> Desligamento { get { return ConsultaOcorrencias(desligamento); } }
    public static List<EventLogEntry> Reinicializacao { get { return ConsultaOcorrencias(reinicializacao); } }

    public static List<EventLogEntry> ConsultaOcorrencias(long instanceId)
    {
        List<EventLogEntry> resultado = new List<EventLogEntry>();

        if (EventLog.Exists("System"))
        {
            using (var log = new EventLog("System", Environment.MachineName, "EventLog"))
            {
                var entries = new EventLogEntry[log.Entries.Count];
                log.Entries.CopyTo(entries, 0);
                resultado = entries.Where(x => x.InstanceId == instanceId).ToList();
            }
        }

        return resultado;
    }
}

Main

static void Main(string[] args)
{


    var ultimaInicializacao = ConsultaEventosHelper.Inicializacao.LastOrDefault();
    var ultimoDesligamento = ConsultaEventosHelper.Desligamento.LastOrDefault(); 
    var ultimaReinicializacao = ConsultaEventosHelper.Reinicializacao.LastOrDefault(); 

    Console.WriteLine("Última Incializacao: {0} - {1}", ultimaInicializacao.TimeGenerated, ultimaInicializacao.Message);
    Console.WriteLine("Última Reinicializacao: {0} - {1}", ultimaReinicializacao.TimeGenerated, ultimaReinicializacao.Message);
    Console.WriteLine("Último Desligamento: {0} - {1}", ultimoDesligamento.TimeGenerated, ultimoDesligamento.Message);


    Console.ReadKey();
}

Using a different approach to get the TimeGenerated .

  

Note: Generally, there is a delay between the time an event is generated and the time at which it is logged. It is more important to know when the event was generated, unless you want to see if there is a significant delay in the log. This can happen if your log files are on a different server and you have a bottleneck. Source: link

 static void Main(string[] args)
        {
            EventLog ev = new EventLog("System", Environment.MachineName);

            if (ev.Entries.Count <= 0)
                Console.WriteLine("Nenhum log de eventos no log :" + "System");

            for (int i = ev.Entries.Count - 1; i >= 0; i--)
            {
                EventLogEntry CurrentEntry = ev.Entries[i];

                DateTime dt = DateTime.Now;
                TimeSpan ts = dt.Subtract( CurrentEntry.TimeGenerated);
                int hours = (ts.Days * 24) + ts.Hours;

                if (CurrentEntry.Source.ToUpper() == "USER32")
                {
                    Console.WriteLine("Tempo gerado:" + CurrentEntry.TimeGenerated);
                    Console.WriteLine("Horas atrás:" + hours);
                    Console.WriteLine("Event Id : " + CurrentEntry.InstanceId);
                    Console.WriteLine("Entry Type : " + CurrentEntry.EntryType.ToString());
                    Console.WriteLine("Mensagem :  " + CurrentEntry.Message + "\n");
                }
            }
            ev.Close();
            Console.ReadLine();
        }

Source code here.

You can control at least logoff and shutdown as follows:

using Microsoft.Win32;

namespace MyApp
{
    public MyClass()
    {
        SystemEvents.SessionEnding += SystemEvents_SessionEnding;
    }

    private void SystemEvents_SessionEnding(object sender, SessionEndingEventArgs e)
    {
        if (e.Reason == SessionEndReasons.Logoff)
        {
            // colocar aqui o código para logoff
        }
        else
        {
            // colocar aqui o código para shutdown
        }
    }
}