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ASP.NET MVC 3 - Setting null field in DTO when binding to HTTP POST instead of failing
[HttpPost]
public ViewResult Index(ResultQueryForm queryForm)
{
   ...
}

public class ResultQueryForm
{
   public DateTime? TimestampStart { get; set; }
   public DateTime? TimestampEnd { get; set; }
   public string Name { get; set; }
}

[HttpPost]
public ViewResult Index(DateTime? startDate)
{
   // If the user enters an invalid date, the controller action won't even be run because   the MVC model binding will fail and return an error message to the user
}

if(!Model.IsValid)
{
  return View(); // ooops didn't work
}
else
{
  return RedirectToAction("Index"); //horray
}

[DateTimeFormat(ErrorMessage = "Invalid date format.")]
public DateTime? TimestampStart { get; set; }
[DateTimeFormat(ErrorMessage = "Invalid date format.")]
public DateTime? TimestampEnd { get; set; }


[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class DateTimeFormatAttribute : ValidationAttribute
{
    public override bool IsValid(object value) {

        // allow null values
        if (value == null) { return true; }

        // when value is not null, try to convert to a DateTime
        DateTime asDateTime;
        if (DateTime.TryParse(value.ToString(), out asDateTime)) {
            return true; // parsed to datetime successfully
        }
        return false; // value could not be parsed
    }
}