If I have in my model class a property of type DateTime how can I render it in a specific format - for example in the format which ToLongDateString() returns?

I have tried this...

@Html.DisplayFor(modelItem => item.MyDateTime.ToLongDateString())

...which throws an exception because the expression must point to a property or field. And this...

@{var val = item.MyDateTime.ToLongDateString();
  Html.DisplayFor(modelItem => val);
}

...which doesn't throw an exception, but the rendered output is empty (although val contains the expected value, as I could see in the debugger).

Thanks for tips in advance!

Edit

ToLongDateString is only an example. What I actually want to use instead of ToLongDateString is a custom extension method of DateTime and DateTime?:

public static string FormatDateTimeHideMidNight(this DateTime dateTime)
{
    if (dateTime.TimeOfDay == TimeSpan.Zero)
        return dateTime.ToString("d");
    else
        return dateTime.ToString("g");
}

public static string FormatDateTimeHideMidNight(this DateTime? dateTime)
{
    if (dateTime.HasValue)
        return dateTime.Value.FormatDateTimeHideMidNight();
    else
        return "";
}

So, I think I cannot use the DisplayFormat attribute and DataFormatString parameter on the ViewModel properties.