ssrs =DateDiff("ww",Fields!date.Value,today) cannot compare data of types system.int64 system.string

hi,

 i am trying to filter the matrix and trying to compare the difference between two date values and getting the following error.

cannot compare data of types system.int64 system.string

here is my filter expression =DateDiff("ww",Fields!date.Value,today)

datetime datatype following is the value of the field

fields!date.Value= '2010-07-16 10:22:53.093'

 please let me know what i am doing wrong here?? how to resolve this?

thanks