What is the expression to get the count of blank columns for the above column in my requirement?

Hi,

    I want to get the count of the blank columns for each date group in the below image.

pl. have a look at the below image which describes my requirement.

I have tried with the below Expression which gives "7" as count for all the columns, which is wrong. 

=Sum(IIF(notISNOTHING(Fields!TimeSheetStatus.Value),1,0),"CostCentre")

and if use the below Expression its giving an error

=Sum(IIF(IsNothing(ReportItems!TimeSheetStatus.Value),1,0))

ERROR:

• An error occurred during local report processing.
  • The definition of the report 'TS1.rdlc' is invalid.
    • The Value expression for the textrun 'Textbox34.Paragraphs[0].TextRuns[0]' uses an aggregate function on a report item.  Aggregate functions can be used only on report items contained in page headers and footers.

Thanks in advance,

kranthi kumar G.