Got caught out myself on this little issue, but used this to work around it :
Format([Fieldname],"ww/yy- ",0)
or
if you want to make it so it appears in the right order:
Week: IIf(Len(Format([Fieldname],"ww"))=1,Format([Fieldname],"-yy-" & "0" & "ww"),Format([Fieldname],"-yy-ww"))
The...