ValidationMessage asp.net mvc

I have a question about ModelState.AddModelError method and about the ValidationMessage method.

I am new to ASP.NET MVC and I am a little confused.

I wrote this code:

public ActionResult update(FormCollection collection)
{
    int oos = 0;

    try
    {
        oos = int.Parse(collection[0]);
    }
    catch
    {         
     }

    data d = new data();

    TryUpdateModel(d , collection.ToValueProvider());

     if (ModelState.IsValid)
    { 
         return View("index",d);
    }
    else
    {      
         ModelState.AddModelError("Date", "Wronge Date");   
         d.Id = 50;
        return View("index",d);
    }
}

and this code in the view side

@{

    ViewBag.Title = "index";
}

<h2>index</h2>
@TempData["Hi"]


@Html.ValidationMessage("fullname") 

@using (Html.BeginForm()) 
{ 
    @Html.AntiForgeryToken() @Html.TextBox("id", 70) 
    @Html.TextBox("Date", "3/2/1991 12:00:00 ص")
    @Html.ValidationMessage("Date","Please insert the correct Date Format") 

    <input type="submit"> 
}

My questions are, why the message Please insert the correct Date Format appears directly when I rune the index while still I did not submit the index, why when I submit the form with error in the date format the same message appear but not the message that I set to the Date Key in the update method which is Wronge Date.

maybe still I do not understand those two methods so I hope to find somebody to explain them to me.

explnation with example or reference would be appreciated