Use AddDays function in vb.net - vb.net

Question

Need some help finishing up this program, everything works and runs like I want it but I need to display an inputbox that allows the user to enter in the date they want for their invoice stored in service_date and then this date will display in the listbox with all the other items that I have put in there. I know I need to use the AddDays function but I have no clue on how to do it, and researching online has just led me to 100 other things that aren't that.
So here is my code:
Dim Customer As String
Dim Phone As String
Dim Hours As Double
Dim Parts As Double
Dim due_date As String
Dim service_date As String
Private Sub cmdInputBox_Click()
Dim service_date = InputBox("Enter the date of service. (MM/DD?YYYY)")
MsgBox("That's your date, " & service_date.ToString)
Exit Sub
End Sub
Private Sub CustInfo_Click()
Customer = txtCustomer.Text
Phone = mtbPhone.Text
Double.TryParse(txtHours.Text, Hours)
Double.TryParse(txtParts.Text, Parts)
If Customer.Length < 0 Then
MessageBox.Show("Please enter customer information.")
End If
If Phone = "" Then
MessageBox.Show("Please enter phone number.")
End If
If Not Double.TryParse(txtHours.Text, Hours) Then
MessageBox.Show("Please enter labor hours.")
End If
If Not Double.TryParse(txtParts.Text, Parts) Then
MessageBox.Show("Please enter parts and supplies.")
End If
''Perform calculations
Dim Total_Cost As Double
Dim Labor_Cost As Double
Dim Parts_Cost As Double
Parts_Cost = (Parts * 0.5 * 2)
Labor_Cost = (Hours * 35)
Total_Cost = (Hours + Parts)
Customer = txtCustomer.Text
Phone = mtbPhone.Text
lstBill.Items.Clear()
lstBill.Items.Add("Customer: " & vbTab & Customer.ToUpper)
lstBill.Items.Add("Phone: " & vbTab & vbTab & Phone)
lstBill.Items.Add("Service Date: " & vbTab & due_date)
lstBill.Items.Add("Invoice Date: " & vbTab & service_date)
lstBill.Items.Add("Labor Cost: " & vbTab & FormatCurrency(Labor_Cost))
lstBill.Items.Add("Parts Cost: " & vbTab & FormatCurrency(Parts_Cost))
lstBill.Items.Add("Total Cost: " & vbTab & FormatCurrency(Total_Cost))
Exit Sub
End Sub
Private Sub btnBill_Click(sender As System.Object, e As System.EventArgs) Handles btnBill.Click
cmdInputBox_Click()
CustInfo_Click()
End Sub

Answer 1

Try this:
Dim strDate As String = InputBox("Enter date?", , "")
If strDate = "" Then Exit Sub
Dim dteDate As Date
Dim enUS As New System.Globalization.CultureInfo("en-US")
If Date.TryParseExact(strDate, "MM/dd/yyyy", enUS, Globalization.DateTimeStyles.AssumeLocal, dteDate) Then
MsgBox("Date is " & dteDate.ToString)
End If