MVC fields not required but user being told they are - asp.net-mvc-4

Question

Title says it all. The model does not require the StartDate field but on POST I'm told it's required. It's one of several search fields, each one optional. Due to that, I'm not checking IsModel.Valid so the search works anyway, but the message shows up onscreen. If I set, in the view, #Html.ValidationSummary(true), that hides the message but the field still turns red.
Also, I do have a check to make sure EndDate is later than StartDate, so I need the messages for errors /requried fields to show up, just not when there ISN'T an error.
Here's the code:
MODEL (Partial)
[Display(Name = "Start Date")]
[DataType(DataType.Date)]
public DateTime StartDate { get; set; }
[Display(Name = "End Date")]
[GreaterThanOrEqualTo("StartDate", ErrorMessage = "End Date must be later than Start Date")]
[DataType(DataType.Date)]
public DateTime EndDate { get; set; }
VIEW (partial)
#using (Html.BeginForm()){
#Html.ValidationSummary(false)
<table>
<tr>
<td>
#Html.DisplayNameFor(m => m.StartDate)
#Html.TextBox("StartDate", "", new { #class = "datefield" })
etc.
SHARED/DISPLAY TEMPLATES
#model Nullable<DateTime>
#(Model != null ? string.Format("{0:M/d/yyyy}", Model) : string.Empty)
SHARED/EDITOR TEMPLATES
#model Nullable<DateTime>
#{
DateTime dt = DateTime.Now;
if (Model != null)
{
dt = (System.DateTime) Model;
}
#Html.TextBox("", String.Format("{0:M/d/yyyy}", dt.ToShortDateString()), new { #class = "datefield", type = "date" })
}
Some of these editors are to make a pop-up calendar work, btw.
I've tried turning on/off various things and one way or another, it still says the date fields are required. Any ideas? Thanks.

Answer 1

Easy way to remove validation is make int Null-able, I have already tested and it works fine. here is example:
public int? Id { get; set; }

Answer 2

As mentioned in the comments, value types like DateTime, int, decimal, etc. are treated as required if you don't make them nullable.
If the GreaterThanOrEqualTo attribute doesn't come from a library (such as MVC Foolproof Validation), you should let it return true if both Startdate en Enddate are null. Else you woud have to write your own custom validation attribute, but it's not that hard to do.