Lag and Lead options

[GreekGod80]

Hi Guys,
I am trying to find a difference between 2 rows in a same column, I have tried using lag function (Cognos report under filters)it didnt work in Cognos, however it does work in SQL server....here is the example for a data

a                    new column
b       difference of 'a' and 'b' = 'c' hours

so if there is 300 rows of data there should be 150 differences.
Kindly Help ...Thanks in Advance
I also have another question if anyone of you know.....under the filters if we have red exclaimation mark, what does that suppose to mean ?
Thanks a Bunch

[blom0344]

Did  you look at the running-difference  function?

[GreekGod80]

Hi,
Thanks blom0344 for the reply. I have tried "running-difference" before my post, it worked well with the Number format. However, I have to use this option for a date formatted field.  ???.... In Cognos I am Getting usual difference in the following form.
21days 13Hours 24Minutes 12Seconds. (Running Difference is not working with that :() Kindly let me know how I can make this thing work, and what If I have to change the " 21days 13Hours 24Minutes 12Seconds" in a regular format like 1.603 which mean one sixth of a day .
Thanks in Advance, Your help will be highly appreciated :)