Using a DatetimePicker the user gets an unhandled exception when selecting any day in the month of December, 9999.  I know this probably never happens, but in our system we use DateTime.MaxValue (12/31/9999 11:59:59) in several areas to indicate that the item (rule, report, etc.) has no expiration date.  So when loading the screen the value 12/31/9999 is displayed in the date time picker, but once the user clicks the drop down of the control they get an exception:

The added or subtracted value results in an un-representable DateTime.
Parameter name: months

[ArgumentOutOfRangeException: The added or subtracted value results in an un-representable DateTime.
Parameter name: months]
   System.DateTime.AddMonths(Int32 months) +4617044
   Gizmox.WebGUI.Forms.MonthCalendar.Render(IContext objContext, IResponseWriter objWriter, Int64 lngRequestID) +302
   Gizmox.WebGUI.Forms.Control.RenderControl(IContext objContext, IResponseWriter objWriter, Int64 lngRequestID) +278
   Gizmox.WebGUI.Forms.Form.Render(IContext objContext, IResponseWriter objWriter, Int64 lngRequestID) +112
   Gizmox.WebGUI.Forms.Form.RenderControl(IContext objContext, IResponseWriter objWriter, Int64 lngRequestID) +365
   Gizmox.WebGUI.Forms.Form.RenderForms(IContext objContext, IResponseWriter objWriter, Int64 lngRequestID) +70
   Gizmox.WebGUI.Forms.Form.RenderControl(IContext objContext, IResponseWriter objWriter, Int64 lngRequestID) +365
   Gizmox.WebGUI.Forms.Form.Gizmox.WebGUI.Common.Interfaces.IForm.RenderForm(IContext objContext, IResponseWriter objWriter, Int64 lngRequestID) +41
   Gizmox.WebGUI.Server.Content.ProcessRequest(HttpContext objHttpContext) +2705
 

I will send a sample application to your support email address, thanks,

Andrew