The problem is conversion from string to type date is not valid - vb.net

Question

Option Explicit On
Imports System.Data.OleDb
Public Class Form1
Dim objCon As New OleDbConnection
Dim strSQL As String
Dim strConnect As String = "Provider= Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Abry\Documents\Bilik.accdb"
Dim da As New OleDb.OleDbDataAdapter
Dim ds As New DataSet
Private Sub TempahButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TempahButton.Click
Dim startDate As String = DateTimePicker1.Value.ToString("dd/MM/yyyy")
Dim str As String
startDate = DateTimePicker1.Value
objCon.ConnectionString = strConnect
objCon.Open()
str = "Insert into bilik(Bilik, Tujuan, [Masa Masuk], [Masa Keluar]) values(?, ?, ?, ?)"
str = "Insert into bilik(Tarikh) values(startDate = DateTimePicker1.Value.ToShortDateString)"
Dim cmd As OleDbCommand = New OleDbCommand(str, objCon)
cmd.Parameters.Add(New OleDbParameter("Bilik", CType(BilikComboBox.SelectedIndex, String)))
cmd.Parameters.Add(New OleDbParameter("Tujuan", CType(TujuanTextBox.Text, String)))
cmd.Parameters.Add(New OleDbParameter("Tarikh", CType(DateTimePicker1.Value, Date)))
cmd.Parameters.Add(New OleDbParameter("Masa Masuk", CType(MasaMasukMaskedTextBox.Text, String)))
cmd.Parameters.Add(New OleDbParameter("Masa Keluar", CType(MasaKeluarMaskedTextBox.Text, String)))
Try
cmd.ExecuteNonQuery()
cmd.Dispose()
objCon.Close()
TujuanTextBox.Clear()
BilikComboBox.Text = ""
DateTimePicker1.Value = ""
MasaMasukMaskedTextBox.Clear()
MasaKeluarMaskedTextBox.Clear()
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
End Class
Tarikh is date declare
Conversion from string to type date is not valid

Answer 1

The issue is, apparently, here:
DateTimePicker1.Value = ""
That means that everything else you posted is irrelevant. The issue is, as the error message clearly states, it is not valid to assign an empty String to a property that is type Date.
The Value property of a DateTimePicker is type Date and so you must assign a Date value to it. There's no such thing as an empty Date so you must either assign a default date, e.g. today's date:
DateTimePicker1.Value = Date.Today
or you need to use the functionality built into the DateTimePicker control to indicate that no date is selected. If you want to know how a DateTimePicker control works, read the class documentation.