This the VB version:
Private Sub DateTimePicker1_ValueChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DateTimePicker1.ValueChanged
Dim d As DateTime = DateTimePicker1.Value
Dim i As Integer = d.Minute Mod 15
If (i > 0) Then
DateTimePicker1.Value = DateTimePicker1.Value.AddMinutes(15 - i).AddSeconds(-d.Second)
End If
End Sub