You need to enter it in mask format.
Examples:
MaskedTextBox1.Text = "00/00/0000"
MaskedTextBox1.Text = "01/06/1998"
MaskedTextBox1.Text = "10/15/2013"
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Edit:
To put the system date into the masked text box use this code
Dim formated_date = My.Computer.Clock.GmtTime.ToShortDateString
If formated_date.Split("/")(0).Length = 1 Then
formated_date = "0" & formated_date.ToString
End If
If formated_date.Split("/")(1).Length = 1 Then
formated_date = formated_date.Replace("/" & My.Computer.Clock.GmtTime.Day & "/", "/0" & My.Computer.Clock.GmtTime.Day & "/")
End If
MaskedTextBox1.Text = formated_date