Notes/Domino 4 and 5 Forum

I'm trying to get the year of a DateTime returned as a string, but I get a type mismatch on the 4th line below...

Dim thisDate As NotesDateTime
Dim yearStr As String
Set thisDate = New NotesDateTime( Today )
yearStr = Cstr(Year(thisDate))

I don't understand this. Doesn't Year() return an integer, and Cstr() convert integers to strings?

Note, however, that the following DOES work...

yearStr = Cstr(Year( Today ))

But why can't I use my DateTime in place of Today? (In the actual application, my thisDate could be any date. It's not actually going to be Today.)