Today is my first venture into Xpath..
I have a table that looks like this

Date Level 1 Level 2
05/20/2009 Value1
05/27/2009 Value2

Whenever date for value1 in level 1 changes date for value 2 in level 2 must change consistently by a week. that is if date for value 1 changes to 05/22/2009 then date for value 2 must change to 05/29/2009. How can I accomplish this using xpath expression?

As is, you cannot. First of all you need to convert your data in an XML format. Then perhaps we can help you.