>What is this use of question mark in a parameter:
>
>

>Private Function GetNonNullable(ByVal tdNullable As Date?) As Date
>

I'd say it means that tdNullable can be null http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx#Y1537

You may want to test tdNullable.HasValue to verify whether it contains a value