Public Function GetFolderCreationTimeLimits(ByVal folder As String, _
                                              ByRef mindate As Date, _
                                              ByRef maxdate As Date) As Boolean
 
        mindate = New Date(2100, 12, 31)
        maxdate = New Date(1900, 1, 1)
 
        If Not IO.Directory.Exists(folder) Then Return False
        Try
            For Each file As String In IO.Directory.GetFiles(folder)
                Dim ct As Date = New IO.FileInfo(file).CreationTime
                If mindate > ct Then mindate = ct
                If maxdate < ct Then maxdate = ct
            Next file
            Return True
        Catch
            Return False
        End Try
  End Function

Public Function GetNewestOldestFolderItem(ByVal pFolder As String, ByRef _
  pOldest As Date, ByRef pNewest As Date) As Boolean
 
        If Directory.Exists(pFolder) Then
 
            Dim dirinf As New DirectoryInfo(pFolder)            
            Dim files = dirinf.GetFiles().OrderBy(Function(x) x.CreationTime)
            If files.Count > 0 Then
                pNewest = files(0).CreationTime
            Else
                Return False
            End If
            Dim filesdesc = dirinf.GetFiles().OrderByDescending(Function(x) _
              x.CreationTime)
            If filesdesc.Count > 0 Then
                pOldest = filesdesc(0).CreationTime
            Else
                Return False
            End If
 
            Return True
        Else
            Return False
        End If
 
    End Function

   Public Function GetFolderCreationTimeLimits2(ByVal folder As String, _
                                            ByRef mindate As Date, _
                                            ByRef maxdate As Date) As Boolean
 
        Try
            Dim dinf As New IO.DirectoryInfo(folder)
            mindate = dinf.GetFiles.Min(Function(x) x.CreationTime)
            maxdate = dinf.GetFiles.Max(Function(x) x.CreationTime)
            Return True
        Catch
            Return False
        End Try
 
    End Function

GetFolderCreationTimeLimits(TextBox5.Text, mindate:=Label1.Text, _
  maxdate:=Label2.Text)

Dim mindate, maxdate as date
 
If not GetFolderCreationTimeLimits(TextBox5.Text, mindate, maxdate) then
   msgbox("Fehler")
   Label1.text = ""
   Label2.text = ""
Else
   Label1.text = mindate.tostring
   Label2.Text = maxdate.tostring
End If

Dim von As String = Label1.Text.Substring(16, 8)
Dim bis As String = Label2.Text.Substring(16, 8)

Dim von As String = "12:17:35"         
Dim bis As String = "19:12:10" 
 
Dim diff As String = _
     (New TimeSpan(CDate(bis).Ticks - CDate(von).Ticks)).ToString()

Public Function GetFolderCreationTimeLimits(ByVal folder As String, ByVal _
  suchtag As Date, ByRef mindate As Date, ByRef maxdate As Date) As Boolean
 
        Try
            Dim dinf As New IO.DirectoryInfo(folder)
            suchtag = TextBox2.Text
            mindate = dinf.GetFiles.Min(Function(x) x.CreationTime)
            maxdate = dinf.GetFiles.Max(Function(x) x.CreationTime)
            Return True
        Catch
            Return False
        End Try
 
    End Function

Public Shared Function GetFolderCreationTimeLimits3 _
                      (ByVal folder As String, _
                       ByVal criticalday As Date, _
                       ByRef mintime As Date, _
                       ByRef maxtime As Date) As Boolean
 
    mintime = New Date(2100, 12, 31)
    maxtime = New Date(1900, 1, 1)
 
    If Not IO.Directory.Exists(folder) Then Return False
 
    Try
       For Each file As String In IO.Directory.GetFiles(folder)
          Dim fi As New IO.FileInfo(file)
          fi.Refresh() 'in VB-Doku empfohlen !!
          Dim ct As Date = fi.CreationTime
          If ct.Date = criticalday.Date Then
             If mintime > ct Then mintime = ct
             If maxtime < ct Then maxtime = ct
          End If
       Next file
       Return True
    Catch
       Return False
    End Try
 
End Function

Dim mintime, maxtime As Date
 
lblCriticalday.Text = "27.3.2015" 'Zeichenfolge in Label-Eigenschaft
 
Dim folder As String = "C:\daten" 'zu durchsuchender Ordner
 
Dim criticalday As Date = CDate(lblCriticalday.text) 'Umwandlung in Date
 
If Not GetFolderCreationTimeLimits3 _
            (folder, criticalday, mintime, maxtime) Then
   MsgBox("Fehler")
Else
   lblMintime.Text = mintime.ToString("g") 'Formatierung von Date als String
   lblmaxtime.Text = maxtime.ToString("g")
End If