Private Function MonthsInTimespan(intervalstart As Date, intervalend As Date) _
    As String()
 
        If intervalstart.Ticks > intervalend.Ticks Then
            Throw New ArgumentException("No Timespan")
        End If
 
        Dim months As New List(Of String)
        If intervalstart.Year = intervalend.Year Then
            'Beide Datumsangaben im gleichen Jahr ... 
            Dim i As Integer = intervalstart.Month
            months.Add(DateAndTime.MonthName(i) & " " & DateTime.DaysInMonth( _
              intervalstart.Year, intervalstart.Month) - intervalstart.Day)
 
            For y As Integer = intervalstart.Month + 1 To intervalend.Month - 1
                months.Add(DateAndTime.MonthName(y) & " " & _
                  DateTime.DaysInMonth(intervalend.Year, y))
            Next
 
            Dim z As Integer = intervalend.Month
            months.Add(DateAndTime.MonthName(z) & " " & DateDiff("d", _
              DateSerial(intervalend.Year, intervalend.Month, 0), intervalend))
 
        ElseIf intervalstart.Year + 1 = intervalend.Year Then
            'Zwischen den Datumsangaben liegt ein Jahreswechsel
            Dim i As Integer = intervalstart.Month
            months.Add(DateAndTime.MonthName(i) & " " & DateTime.DaysInMonth( _
              intervalstart.Year, intervalstart.Month) - intervalstart.Day)
 
            For x As Integer = intervalstart.Month + 1 To 12
                months.Add(DateAndTime.MonthName(x) & " " & _
                  DateTime.DaysInMonth(intervalstart.Year, x))
            Next
 
            For y As Integer = 1 To intervalend.Month - 1
                months.Add(DateAndTime.MonthName(y) & " " & _
                  DateTime.DaysInMonth(intervalend.Year, y))
            Next
 
            Dim z As Integer = intervalend.Month
            months.Add(DateAndTime.MonthName(z) & " " & DateDiff("d", _
              DateSerial(intervalend.Year, intervalend.Month, 0), intervalend))
        Else
            Throw New ArgumentException("Not implemented")
        End If
 
        ListBox1.DataSource = months
 
        Return months.ToArray
 
 
    End Function

Dim Startdatum As Date = "15.03.2018"
        Dim Enddatum As Date = "25.04.2019"
        Dim m() = MonthsInTimespan(CDate(Startdatum), CDate(Enddatum))