newB: can't format date UK to US [working]

[subegt]

I am still new to VB and .NET
I have searched this forum and tried lots of code and nothing works. Here is what I want to do and what I have:

user enters date into textbox1.Text as 28/4/2003
my code should format it to 4/28/2003 and test if isDate

'Check if it is a valid date.
Dim dtDate As Date = textbox1.Text
Dim strDue As String
strDue = Format(dtDate, "MM/dd/yyyy") 'this should format

If IsDate(strDue) = False Then
lblMessage.Text = "Please enter valid due date."
lblMessage.Visible() = True
txtDueDate.Text = ""
Label1.Text = strDue
Label1.Visible = True
Exit Sub
End If

The code above returns an error:
Cast from string "28/4/2003" to type 'Date' is not valid.

If I make dtDate As String then
strDue = Format(dtDate, "MM/dd/yyyy")
returns MM/dd/yyyy in Label1.Text
I used the Label1 to see what the return value of Format is.

Please help me understand what is happening!