jQuery Grid Plugin - jqGrid

To begin with… love the product!

Tougher question.  I need to add dates from a previous row and field. Example:

I want row 2 to update the start date by adding 3 months to 1/2012 result of 4/2012 and then row 3 to update 6 months from 4/2012 etc etc.  A continuation from the previous date/data.  

Or maybe an easier way to ask the question, can you put formulas in the update or cell value kind of like excel.  
B3=B2+C2 and B4=B3+C3 etc etc.

I saw in other posts that it does not do any calculations... I can do the formulas for the data but how can I populate the update field or insert field with the data?

Any help or advice would be greatly appreciated.