Private Structure contract
        Dim von As DateTime
        Dim bis As DateTime
        '........... weitere Vertragsinfos
    End Structure
 
'In einem Modul:
 
 
        Dim Arbeitstage As Integer = 0
 
        Dim contracts As New System.Collections.Generic.List(Of contract)
 
        Dim contract1 As contract
        contract1.von = CDate("15.3.2010")
        contract1.bis = CDate("27.8.2010")
        contracts.Add(contract1)
 
        Dim contract2 As contract
        contract2.von = CDate("15.4.2010")
        contract2.bis = CDate("27.10.2010")
        contracts.Add(contract2)
 
        Dim aktuell As DateTime = CDate("31.12.2009")
        Dim IstArbeitstag(365) As Boolean
        For i As Integer = 1 To 365
            aktuell = aktuell.AddDays(1)
            For k As Integer = 0 To contracts.Count - 1
                If TagInIntervall(aktuell, contracts(k).von, contracts(k).bis) _
                  Then
                    IstArbeitstag(i) = True
                End If
            Next k
        Next i
 
        For i As Integer = 1 To 365
            If IstArbeitstag(i) Then Arbeitstage += 1
        Next i
 
'==========================================================
 
    Public Function TagInIntervall(ByVal aktuell As DateTime, _
                                   ByVal von As DateTime,
                                   ByVal bis As DateTime) As Boolean
 
        Dim ts As New TimeSpan(0)
        If aktuell.Subtract(von) >= ts And _
            aktuell.Subtract(bis) <= ts Then Return True
        Return False
    End Function