Public Class Form1

    Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click
        Dim str = "1/4/19" ' test data '

        If cbDateFormat.SelectedIndex = -1 Then
            MsgBox("Please select the date format.")
            Exit Sub
        End If

        Dim dateFormat = ""

        Select Case cbDateFormat.Text
            Case "DD/MM/YYYY"
                dateFormat = "d/M/yyyy"
            Case "MM/DD/YYYY"
                dateFormat = "M/d/yyyy"
            Case "DD/MM/YY"
                dateFormat = "d/M/yy"
            Case "MM/DD/YY"
                dateFormat = "M/d/yy"
            Case Else
                Throw New Exception("Date format not found.")
        End Select

        Dim d As DateTime
        If DateTime.TryParseExact(str, dateFormat, Globalization.CultureInfo.InvariantCulture, Nothing, d) Then
            ' d now holds the datetime '
            MsgBox(d.ToString("dd-MMM-yyyy"))
        End If

    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        cbDateFormat.Items.AddRange({"DD/MM/YYYY", "MM/DD/YYYY", "DD/MM/YY", "MM/DD/YY"})

    End Sub

End Class