vb.net字符串格式轉爲日期型

vb.net字符串格式轉爲日期型  好比 "20080815" 轉換爲"2008-05-15"
Dim a As Date 
Dim s As String = "20080815" 
a = CDate(Format(CInt(s), "0000-00-00")) 
MsgBox(a.Year)