Visual Basic

Dim subStr As Integer<br />
   If Me.ComboBox2.Text.Contains("jan") Then<br />
       subStr = Me.ComboBox2.Text.Substring(Me.ComboBox2.Text.IndexOf("n") + 1, 3)<br />
       Me.TextBox1.Text = DateTime.DaysInMonth("20" & subStr, 1)<br />
   ElseIf Me.ComboBox2.Text.Contains("feb") Then<br />
       subStr = Me.ComboBox2.Text.Substring(Me.ComboBox2.Text.IndexOf("b") + 1, 3) '1. note the change in the letter within the index of<br />
       Me.TextBox1.Text = DateTime.DaysInMonth("20" & subStr, 2) '2. note the change in the second part of this daysinmonth.<br />
   End If

'Date to test
Dim stringDate As String = "sep 07"
'Date object
Dim parsedDate As Date

'If our string is valid date show days in month
If Date.TryParse(stringDate, parsedDate) Then
    MsgBox(Date.DaysInMonth(parsedDate.Year, parsedDate.Month))
End If

Private Function Days(ByVal text As String) As Integer
    Dim split As String() = text.Split

    'An error could occur so catch it and return -1
    Try
        Dim month As Integer = [Enum].Parse(GetType(Month), split(0).ToUpper)
        Dim year As Integer = CInt("20" & split(1))
        Return Date.DaysInMonth(year, month)
    Catch ex As Exception
        Return -1
    End Try
End Function

Private Enum Month
    JAN = 1
    FEB
    MAR
    APR
    MAY
    JUN
    JUL
    AUG
    SEPT
    OCT
    NOV
    DEC
End Enum