TimedTask.cs

private System.Threading.Timer timer;
private void SetUpTimer(TimeSpan alertTime)
{
     DateTime current = DateTime.Now;
     TimeSpan timeToGo = alertTime - current.TimeOfDay;
     if (timeToGo < TimeSpan.Zero)
     {
        return;//time already passed
     }
     this.timer = new System.Threading.Timer(x =>
     {
         this.SomeMethodRunsAt1600();
     }, null, timeToGo, Timeout.InfiniteTimeSpan);
}

private void SomeMethodRunsAt1600()
{
    //this runs at 16:00:00
}

// usage
var timed = SetUpTimer(new TimeSpan(23, 45, 00));