To reproduce: set the ThemeName property before setting the RadDateTimePickerCalendarShowTimePicker property to true.
    Private Sub RadForm1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        Me.RadDateTimePicker1.ThemeName = "TelerikMetro"
        TryCast(Me.RadDateTimePicker1.DateTimePickerElement.CurrentBehavior, RadDateTimePickerCalendar).ShowTimePicker = True
    End Sub

Workaround:

    Private Sub RadForm1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        TryCast(Me.RadDateTimePicker1.DateTimePickerElement.CurrentBehavior, RadDateTimePickerCalendar).ShowTimePicker = True
        Me.RadDateTimePicker1.ThemeName = "TelerikMetro"
    End Sub