Binding to a WPF custom control dependency property - wpf

Question

I am having a hard time binding a property of a wpf custom control.
Here is an excerp of my xaml custom datagrid cell whose datacontext is the list(of date):
<DataGridTemplateColumn Header="Start" MinWidth="100">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<timePicker:CustomTimePicker selectedTime="{Binding Path=startDate}" MinWidth="100" />
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
As you can see, I try to bind the startDate Property of my List(Of Date) to the selectedTime dependency property in my custom wpf control TimePicker.
Here is the definition of the customtimepicker class with the selectedDate dependency-property:
Public Class CustomTimePicker
Private _hours As String
Private _minutes As String
Private _hoursChoices As List(Of String)
Private _minutesChoices As List(Of String)
Public Shared selectedTimeProperty As DependencyProperty = DependencyProperty.Register("selectedTime", GetType(Date), GetType(CustomTimePicker), New FrameworkPropertyMetadata(DateTime.MinValue, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, New PropertyChangedCallback(AddressOf CustomTimePicker.OnSelectedTimeChanged)))
Public Property selectedTime() As Date
Get
Return DirectCast(GetValue(selectedTimeProperty), Date)
End Get
Set(value As Date)
SetValue(selectedTimeProperty, value)
End Set
End Property
Public Property hours As String
Get
Return _hours
End Get
Set(value As String)
_hours = value
End Set
End Property
Public Property minutes As String
Get
Return _minutes
End Get
Set(value As String)
_minutes = value
End Set
End Property
Public Property hoursChoices As List(Of String)
Get
Return _hoursChoices
End Get
Set(value As List(Of String))
_hoursChoices = value
End Set
End Property
Public Property minutesChoices As List(Of String)
Get
Return _minutesChoices
End Get
Set(value As List(Of String))
_minutesChoices = value
End Set
End Property
Protected Shared Sub OnSelectedTimeChanged(ByVal obj As DependencyObject, ByVal args As DependencyPropertyChangedEventArgs)
Console.WriteLine("#######")
Console.WriteLine(CType(obj, CustomTimePicker).selectedTime.ToString)
End Sub
Protected Shared Sub OnCoerce()
End Sub
Protected Sub OnSelectedHourChanged(obj As Object, e As System.Windows.Controls.SelectionChangedEventArgs) Handles hourComboBox.SelectionChanged
Dim minute As Integer = 0
Dim result As Date = New Date(2000, 1, 1, hourComboBox.SelectedIndex, selectedTime.Minute, 0)
selectedTime = result
Console.WriteLine("---")
Console.WriteLine(result.ToString)
Console.WriteLine(selectedTime.ToString)
End Sub
Protected Sub OnSelectedMinuteChanged(obj As Object, e As System.Windows.Controls.SelectionChangedEventArgs) Handles minutesComboBox.SelectionChanged
Dim minute As Integer = 0
Console.WriteLine("bbbbbb")
Console.WriteLine(selectedTime.ToString)
Select Case minutesComboBox.SelectedIndex
Case 0
minute = 0
Case 1
minute = 15
Case 2
minute = 30
Case 3
minute = 45
End Select
Dim result As Date = New Date(2000, 1, 1, selectedTime.Hour, minute, 0)
selectedTime = result
End Sub
Protected Overrides Sub OnInitialized(e As System.EventArgs)
MyBase.OnInitialized(e)
_hoursChoices = New List(Of String)
_minutesChoices = New List(Of String)
Console.WriteLine("aaaaaaa")
Console.WriteLine(selectedTime.ToString)
For i As Integer = 0 To 23
_hoursChoices.Add(i.ToString)
Next
_minutesChoices.Add("00")
_minutesChoices.Add("15")
_minutesChoices.Add("30")
_minutesChoices.Add("45")
hourComboBox.ItemsSource = hoursChoices
hourComboBox.DisplayMemberPath = hours
hourComboBox.SelectedValuePath = hours
hourComboBox.SelectedValue = hours
minutesComboBox.ItemsSource = minutesChoices
minutesComboBox.DisplayMemberPath = minutes
minutesComboBox.SelectedValuePath = minutes
minutesComboBox.SelectedValue = minutes
Select Case selectedTime.Minute
Case 0 To 14
minutesComboBox.SelectedIndex = 0
Case 15 To 29
minutesComboBox.SelectedIndex = 1
Case 30 To 44
minutesComboBox.SelectedIndex = 2
Case 45 To 59
minutesComboBox.SelectedIndex = 3
End Select
hourComboBox.SelectedIndex = selectedTime.Hour
End Sub
End Class
If I change the selectedTime property for example by
selectedTime = New Date(2000,1,1,23,59,0)
the value does not change in the datagrid. It is the same problem the other way around. I do not get the startTime values to the wpf control.
It really seems like the binding is not working, even though the application compiles and runs error free.
Can somebody help me?
Kind regards