Passing null or zero length string

I've dug around and not found a likely solution to this problem. Perhaps I'm wrong. It's certainly possible :)
Passing onto a stored procedure a value from a GridView. The particular fields are a date type, and numeric type. Problem is, its plausible and conceivable in this case one or both could be null or empty.
Through searching I've seen suggested such ideas as Convert.ToDate(value), CDate(value), and so on. None of these work. You cannot convert an empty string or null value to a date. I don't know about the numeric value.
So, barring any non-working examples already given. Does anyone have any idea? What is standard. Its not like I've stumbled across some never afore seen problem.
Here is my code, but its quite typical.
Dim txtctlHireDate As TextBox = CType(gvEvaluatees.Rows(e.RowIndex).FindControl("txtHireDate"), TextBox)
Dim txtctlGradYear As TextBox = CType(gvEvaluatees.Rows(e.RowIndex).FindControl( ...