Run the attached sample project and try entering "1400/12/30".

            CultureInfo info = new CultureInfo("fa-Ir");

            this.radDateTimePicker1.Format = DateTimePickerFormat.Custom;
            this.radDateTimePicker1.CustomFormat = "yyyy/MM/dd";
            this.radDateTimePicker1.Culture = info;

The following exception occurs: 

   at System.Globalization.PersianCalendar.ToDateTime(Int32 year, Int32 month, Int32 day, Int32 hour, Int32 minute, Int32 second, Int32 millisecond, Int32 era)
   at System.Globalization.Calendar.ToDateTime(Int32 year, Int32 month, Int32 day, Int32 hour, Int32 minute, Int32 second, Int32 millisecond)
   at Telerik.WinControls.UI.ArabicMaskDateTimeProvider.KeyPress(Object sender, KeyPressEventArgs e)
   at Telerik.WinControls.UI.RadMaskedEditBoxElement.TextBoxItem_KeyPress(Object sender, KeyPressEventArgs e)
   at System.Windows.Forms.KeyPressEventHandler.Invoke(Object sender, KeyPressEventArgs e)
   at Telerik.WinControls.RadItem.OnKeyPress(KeyPressEventArgs e)
   at Telerik.WinControls.UI.RadTextBoxItem.TextBoxControl_KeyPress(Object sender, KeyPressEventArgs e)
   at System.Windows.Forms.Control.OnKeyPress(KeyPressEventArgs e)
   at System.Windows.Forms.Control.ProcessKeyEventArgs(Message& m)
   at System.Windows.Forms.Control.ProcessKeyMessage(Message& m)
   at System.Windows.Forms.Control.WmKeyChar(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.TextBoxBase.WndProc(Message& m)
   at System.Windows.Forms.TextBox.WndProc(Message& m)
   at Telerik.WinControls.UI.HostedTextBoxBase.WndProc(Message& message)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at DateTimePickerPersianCalendar.Program.Main() in c:\Projects\DateTimePickerPersianCalendar\DateTimePickerPersianCalendar\Program.cs:line 17
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

To reproduce: 
1. Add RadCalendar/or RadDateTimePicker/ and the MaxValue of calendar control to DateTime.MaxValue.
2. Click on the header and an exception is thrown: System.ArgumentOutOfRangeException: 'The added or subtracted value results in an un-representable DateTime.
Parameter name: months'

Workaround:
To work around this issue we can stop showing this pop-up when one of the three last months of the MaxDate.Year is selected in the RadCalendar ViewChanged event:

Please refer to the attached sample project and run it with RadForm2.

Note: the issue is not reproduced consistently.

By default, if you need to format the cells in RadCalendar, the ElementRender event is an appropriate solution. However, it doesn't process the row/column header cell elements illustrated in the attached screenshot.

Workaround:

How to reproduce:
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();


        ThemeResolutionService.ApplicationThemeName = "VisualStudio2012Light";

        this.radCalendar1.ShowRowHeaders = true;
        this.radCalendar1.AllowMultipleView = true;
        this.radCalendar1.MultiViewRows = 2;
        this.radCalendar1.MultiViewColumns = 2;
        this.radCalendar1.ShowFooter = true;
        this.radCalendar1.ShowOtherMonthsDays = false;
    }
}

Workaround: 
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);

    MultiMonthViewElement viewElement = this.radCalendar1.CalendarElement.CalendarVisualElement as MultiMonthViewElement;
    CalendarMultiMonthViewTableElement tableElement = viewElement.FindDescendant<CalendarMultiMonthViewTableElement>() as CalendarMultiMonthViewTableElement;
    if (tableElement == null)
    {
        return;
    }

    foreach (RadElement child in tableElement.Children)
    {
        MonthViewElement monthViewElement = child as MonthViewElement;
        if (monthViewElement == null)
        {
            continue;
        }


        monthViewElement.FindDescendant<CalendarCellElement>().DrawBorder = false;
    }
}

Workaround:
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();

        this.radDateTimePicker1.DateTimePickerElement.Calendar.ElementRender += Calendar_ElementRender;
    }

    private void Calendar_ElementRender(object sender, RenderElementEventArgs e)
    {
        if (this.radDateTimePicker1.DateTimePickerElement.Calendar.ZoomLevel == ZoomLevel.Days)
        {
            return;
        }

        CalendarCellElement element = e.Element as CalendarCellElement;
        if (element != null)
        {
            element.WeekEnd = false;
        }
    }
}

To reproduce: use the following code snippet:

            radCalendar1.AllowMultipleSelect = true;
            radCalendar1.ShowRowHeaders = true;
            radCalendar1.AllowRowHeaderSelectors = true;
 If you select several cells, they all are selected. Then, click the header row cell. You will notice that the cells from this row are selected but all previously selected cells are cleared. The attached gif file illustrates the behavior. 

Workaround: use the MouseDown event to store the currently selected cells and then on MouseUp restore the missing selection:

public class MyCalendar : RadCalendar
{
    List<DateTime> selectedDays = new List<DateTime>();

    protected override void OnMouseDown(MouseEventArgs e)
    {
        CalendarCellElement cell = this.ElementTree.GetElementAtPoint(e.Location) as CalendarCellElement;
        if (cell != null)
        {
            bool isHeader = (bool)cell.GetValue(CalendarCellElement.IsHeaderCellProperty);
            if (isHeader)
            {
                selectedDays.Clear();
                foreach (DateTime date in this.SelectedDates)
                {
                    selectedDays.Add(date);
                }
            }
        }
        base.OnMouseDown(e);
    }

    protected override void OnMouseUp(MouseEventArgs e)
    {
        base.OnMouseUp(e);
        foreach (DateTime date in selectedDays)
        {
            if (!this.SelectedDates.Contains(date))
            {
                this.SelectedDates.Add(date);
            }
        }
    }

    public override string ThemeClassName  
    { 
        get 
        { 
            return typeof(RadCalendar).FullName;  
        }
    }
}

To reproduce:
public Form1()
{
    InitializeComponent();
    this.radCalendar1.RangeMinDate = new DateTime(2014, 1, 1);
    this.radCalendar1.RangeMaxDate = new DateTime(2017, 7, 17);
    this.radCalendar1.FocusedDate = new DateTime(2014, 2, 4);
    this.radCalendar1.AllowFastNavigation = true;
    this.radCalendar1.FastNavigationStep = 1; 
    this.radCalendar1.AllowMultipleView = true;
    this.radCalendar1.MultiViewColumns = 4;
    this.radCalendar1.MultiViewRows = 3;
    this.radCalendar1.CurrentViewRow = 0;  
    this.radCalendar1.CurrentViewColumn = 1;  
}

The user expects when fast navigating forward, the view to be shifted with one year = 12 months. However, each next forward fast navigation moves with 11 months. 

Workaround: 

            this.radCalendar1.CurrentViewRow = 0;  
            this.radCalendar1.CurrentViewColumn = 0;
            this.radCalendar1.CalendarElement.PreviousButton.PerformClick();

To reproduce: use the following code snippet:

public Form1()
{
    InitializeComponent();

    this.radCalendar1.AllowMultipleView = true;
    this.radCalendar1.MultiViewRows = 3;
    this.radCalendar1.MultiViewColumns = 4;
    this.radCalendar1.AllowMultipleSelect = true;
    this.radCalendar1.ShowRowHeaders = true;
    this.radCalendar1.ShowColumnHeaders = true;
    this.radCalendar1.AllowColumnHeaderSelectors = true;
    this.radCalendar1.AllowRowHeaderSelectors = true;
    this.radCalendar1.ShowViewSelector = true;
    this.radCalendar1.AllowViewSelector = true;

  
    this.radCalendar1.RangeMinDate = new DateTime(2014, 3, 3);
    this.radCalendar1.RangeMaxDate = new DateTime(2014, 7, 27);
    this.radCalendar1.FocusedDate = new DateTime(2014, 5, 5);
}

Please refer to the attached gif file.

Workaround: 
private void Calendar_MouseDown(object sender, MouseEventArgs e)
{
    CalendarCellElement cell = this.radCalendar1.CalendarElement.ElementTree.GetElementAtPoint(e.Location) as CalendarCellElement;
    if (cell != null && cell.Date < this.radCalendar1.RangeMinDate && (bool)cell.GetValue(CalendarCellElement.IsHeaderCellProperty) == true)
    {
        foreach (MonthViewElement monthView in this.radCalendar1.CalendarElement.CalendarVisualElement.Children[0].Children.Last().Children)
        {
            {
                foreach (CalendarCellElement monthCell in monthView.TableElement.Children)
                {
                    if (monthCell.Date.Month == cell.Date.Month && monthCell.Date.Year == cell.Date.Year &&
                        this.radCalendar1.RangeMinDate <= monthCell.Date)// &&
                       // (bool)cell.GetValue(CalendarCellElement.IsHeaderCellProperty) == false)
                    {
                        if (char.IsLetter(cell.Text[0]))
                        {
                            //perform column selection
                            if (monthCell.ControlBoundingRectangle.X == cell.ControlBoundingRectangle.X)
                            {
                                this.radCalendar1.SelectedDates.Add(monthCell.Date);
                            }
                        }
                        else
                        {
                            //perform row selection
                            if (monthCell.ControlBoundingRectangle.Y == cell.ControlBoundingRectangle.Y)
                            {
                                this.radCalendar1.SelectedDates.Add(monthCell.Date);
                            }
                        }
                    }
                }
            }
        }
    }
}

The following line does not move MultiMonthView to the specific date:  

radCalendar1.CalendarElement.View.ViewStartDate = new DateTime(DateTime.Now.Year, 1, 1);


Workaround: use RadCalendar's FocusedDate property instead:

radCalendar1.FocusedDate = new DateTime(2014, 1, 1);

To reproduce: 
1. Drag and drop RadCalendar to form
2. Set design time the AllowMultipleView property to true
3. Set design time value of the MultiViewRows and MultiViewColumns properties to 2
4. When save the form or run the project an exception is thrown. 

Workaround: 
Set the AllowMultipleView and MultiViewColumns properties with code.

To reproduce: 

1. When set the Disabled to true  of special day has not effect
this.radCalendar1.SpecialDays.Add(new RadCalendarDay(new DateTime(2014,7, 15)));
this.radCalendar1.SpecialDays[0].Disabled = true;

2. When set the Selected and Selectable to false, the special day is still can select and highlight
this.radCalendar1.SpecialDays[0].Selected = false;
this.radCalendar1.SpecialDays[0].Selectable = false;

To reproduce: use the following code snippet:

public Form1()
{
    InitializeComponent();
    this.radCalendar1.AllowMultipleView = true;
    this.radCalendar1.MultiViewRows = 3;
    this.radCalendar1.HeaderNavigationMode = Telerik.WinControls.UI.HeaderNavigationMode.Zoom;
}

FIX. RadCalendar - Incorrect week numbering

Use the following code snippet:

radCalendar1.AllowMultipleView = true;
radCalendar1.MultiViewColumns = 1;
radCalendar1.MultiViewRows = 5;
radCalendar1.ShowRowHeaders = true;

CultureInfo ci = new CultureInfo("en-US");
CalendarWeekRule weekRule = CalendarWeekRule.FirstDay;
ci.DateTimeFormat.CalendarWeekRule = weekRule;
radCalendar1.Culture = ci;

Expected behavior: when the CalendarWeekRule is FirstDay, 01/01/2013 should be displayed as week 1

Use the following code snippet:

private void radCalendar1_ViewChanged(object sender, EventArgs e)
{
    this.radCalendar1.SpecialDays.Clear();

    RadCalendarDay day = new RadCalendarDay(new DateTime(2014,04,15));
    radCalendar1.SpecialDays.Add(day);
    PictureBox pictureBox = new PictureBox();
    pictureBox.SizeMode = PictureBoxSizeMode.StretchImage;
    pictureBox.Image = Properties.Resources.home;
    RadHostItem hostItem = new RadHostItem(pictureBox);
    day.TemplateItem = hostItem;
}

Clicking the navigation buttons does not navigate you to the respective month.

Workaround: assign the RadCalendarDay.TemplateItem before adding the day in the SpecialDays collection:
private void radCalendar1_ViewChanged(object sender, EventArgs e)
{
    this.radCalendar1.SpecialDays.Clear();

    RadCalendarDay day = new RadCalendarDay(new DateTime(2014,04,15));
    PictureBox pictureBox = new PictureBox();
    pictureBox.SizeMode = PictureBoxSizeMode.StretchImage;
    pictureBox.Image = Properties.Resources.home;
    RadHostItem hostItem = new RadHostItem(pictureBox);
    day.TemplateItem = hostItem; 
    radCalendar1.SpecialDays.Add(day);
}

Note: if the initial code is used in the ViewChanging event, clicking the navigation buttons leads to freezing the application.

RadCalendar should support month selection. I.e, the end-user should be able to press a single button, and all the cells from the current month should become selected.

Resolution: 

#1 This behavior could be achieved very easy with current API, so we are thinking  that there is no need to implement it in our RadCalendar as embedded functionality. 

For example: 

C#

            this.radCalendar1.AllowMultipleSelect = true;
            for (int i = 1; i <= DateTime.DaysInMonth(DateTime.Now.Year,DateTime.Now.Month); i++)
            {
                this.radCalendar1.SelectedDates.Add(new DateTime(DateTime.Now.Year, DateTime.Now.Month, i));
            }
VB

Me.radCalendar1.AllowMultipleSelect = True
For i As Integer = 1 To DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month)
Me.radCalendar1.SelectedDates.Add(New DateTime(DateTime.Now.Year, DateTime.Now.Month, i))
Next

#2 You can set ShowOtherMonthsDays to false and use ViewSelector. 

For example: 

C# 
this.radCalendar1.AllowMultipleSelect = true;
this.radCalendar1.AllowViewSelector = true;
this.radCalendar1.ShowViewSelector = true;
this.radCalendar1.ShowOtherMonthsDays = false;
VB
Me.radCalendar1.AllowMultipleSelect = True
Me.radCalendar1.AllowViewSelector = True
Me.radCalendar1.ShowViewSelector = True
Me.radCalendar1.ShowOtherMonthsDays = False

Although RadCalendarDay has a Tooltip property, it does not show any tooltips.

To reproduce: select 01/01/0001 and click the previous button