试试
DateTime.ParseExact()
来自MSDN的数据示例：
Dim dateString,format As String
Dim result As Date
Dim provider As Globalization.CultureInfo = Globalization.CultureInfo.InvariantCulture
' Parse date and time with custom specifier.
dateString = "20090910"
format = "yyyyMMdd"
Try
result = Date.ParseExact(dateString,format,provider)
Console.WriteLine("{0} converts to {1}.",dateString,result.ToString())
Console.ReadLine()
Catch e As FormatException
Console.WriteLine("{0} is not in the correct format.",dateString)
End Try