Visual Basic

Public Class Form1
    Dim LastDate As Date = Nothing

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Initialize the LastDate to the current date selected
        'in the DateTimePicker1
        LastDate = DateTimePicker1.Value
    End Sub

    Private Sub DateTimePicker1_ValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DateTimePicker1.ValueChanged
        'Validate the date selected.  If it's past the current date
        'then reset it to the last date selected
        If Date.Compare(DateTimePicker1.Value, Now) > 0 Then
            DateTimePicker1.Value = LastDate
        Else
            LastDate = DateTimePicker1.Value
        End If
    End Sub
End Class

Public Class Form1
    Dim LastDate As Date

    Private Sub DateTimePicker_GotFocus(ByVal sender As Object, ByVal e As System.EventArgs) Handles DateTimePicker1.GotFocus, DateTimePicker2.GotFocus
        LastDate = CType(sender, DateTimePicker).Value
    End Sub

    Private Sub DateTimePicker_ValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DateTimePicker1.ValueChanged, DateTimePicker2.ValueChanged
        Dim dtp As DateTimePicker = DirectCast(sender, DateTimePicker)

        If Date.Compare(dtp.Value, Now) > 0 Then
            dtp.Value = LastDate
        End If
    End Sub
End Class