SharePoint:DateTimeControl Crashes in SP2013 Webpart

Question

I have implemented the SharePoint:DateTimeControl in a very simple webpart

<SharePoint:DateTimeControl ID="dtStartdate" runat="server" DateOnly="true" />

The problem is when I click on the control it renders an exception

Server Error in '/' Application.
Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 
[NullReferenceException: Object reference not set to an instance of an object.]
   Microsoft.SharePoint.Utilities.SPUtility.GetThemedImageUrl(String originalUrl, String themeKey) +199
   Microsoft.SharePoint.WebControls.DatePicker..ctor() +1460
   Microsoft.SharePoint.WebControls.SPDatePickerControl.InitDatePicker() +44
   Microsoft.SharePoint.WebControls.SPDatePickerControl.set_MinJDay(Int32 value) +25
   Microsoft.SharePoint.ApplicationPages.DatePickerFrame.Page_Load(Object sender, EventArgs e) +1708
   System.EventHandler.Invoke(Object sender, EventArgs e) +0
   System.Web.UI.Control.LoadRecursive() +94
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2935

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.17929

Any advice or help to resolve this is highly appreciated, thanks!

== EDIT ==

Codebehind is as follows

[ToolboxItemAttribute(false)]
public partial class BookingWebPart : WebPart
{
    public BookingWebPart()
    {
    }

    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
        InitializeControl();
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        SPList hotelsList = SPContext.Current.Web.Lists["Hotels"];
        foreach (SPListItem item in hotelsList.Items)
        {
            ddlHotels.Items.Add(new ListItem(item.Title, item.ID.ToString()));
        }
        for (int i = 1; i < 15; i++)
        {
            ddlNumberOfNights.Items.Add(i.ToString());
        }
    }

    protected void btnCreateBooking_Click(object sender, EventArgs e)
    {
        SPList bookingsList = SPContext.Current.Web.Lists["Bookings"];
        SPListItem newItem = bookingsList.Items.Add();
        newItem["Hotel"] = ddlHotels.SelectedValue;
        newItem["Startdate"] = dtStartdate.SelectedDate;
        newItem["Enddate"] = dtStartdate.SelectedDate.AddDays(Convert.ToInt32(ddlNumberOfNights.SelectedValue));
        newItem.Update();
        Page.Response.Redirect(bookingsList.DefaultViewUrl);            
    }
}

Answer 1

It seems to be a bug in SharePoint 2013. This is how I solved:

You just need to add the DatePickerFrameUrl attribute to your control declaration

<SharePoint:DateTimeControl ID="dtStartdate" runat="server"
DateOnly="true" DatePickerFrameUrl="<%
$SPUrl:~sitecollection/_layouts/15/iframe.aspx %>" />

Answer 2

If you have this Calendar control in a site collection but not under root collection, you may have to create a root site collection.

For example,

www.xxx.com/ (Root site collection not created) www.xxx.com/sites/site1 (Calendar is in this site collection)

Answer 3

<SharePoint:DateTimeControl ID="dtStartdate" runat="server"
DateOnly="true" DatePickerFrameUrl="<%
$SPUrl:~sitecollection/_layouts/15/iframe.aspx %>" />

This is the correct way in sharepoint2013. thanks. youtou need to specify DatePickerFrameUrl attribute