Compare Rows of the same field ( Date) in webi

Hi,

I have a business req where i need to compare multiple rows of the same field ( date field ) and if the days between 1st row and 2nd row is <30 days, i need to show that record. similarly compare 2nd row with 3 row of the same field and if it falls within 30days range show the 2nd row and etc.

have tried using previous function, but we cannot apply filter on previous funtion , since i need to filter out the records that won't fall under 30 days range.

Please find the eg:

Student ID=1, have 3  records for the same services (Ser1) , i need to compare date : 13/1/2016 with 1/1/2016 and if daysbetween them <30 i need to show the record, similarly compare 23/1/2016 with 13/1/2016 for the same student and if it falls <30 days show the record

How do we achieve this in webi ?

Thanks,

-Varaprasad