SSRS error 'Week' is not a member of Microsoft.VisualBasic.dateInterval

Hi All 

In my below expression i get the error as 

 'Week' is not a member of Microsoft.VisualBasic.dateInterval

=COUNT(IIF(
(DateDiff(DateInterval.Week,Cdate("01/01/1900"),Fields!createdonValue.Value) = DateDiff(DateInterval.Week,Cdate("01/01/1900"),Now())-1)
Or (DateDiff(DateInterval.Week,Cdate("01/01/1900"),Fields!createdonValue.Value) = DateDiff(DateInterval.Week,Cdate("01/01/1900"),Now())-2),Fields!leadid.Value,Nothing))

Any help much appreciated

Thanks

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Pradnya07