Insert a new column after every 7 columns with a specific formula

[Saya_26]

Hi,

I have a spreadsheet containing Employee names, the project they've worked on, and the days. Basically, each day in a week, the employee spends a certain amount of time working on that project, say 5 (hrs). The data that I have contains this detail for all the individual days, the only problem is that it is in a format that has incremented values..

I'm looking for a method which allows me to add a new column after every 7 columns with the new column containing the formula for subtracting the values of the 7th column with the value of the 1st column. Example, G2-A2. So that the new column when created would immediately contain the subtracted value of the two columns. And also if there is any way for me to specify the starting column, since the first few columns will have other data.

Would really appreciate it if anyone has any ideas on how to go about it.

Thanks!

 

[Saya_26]

Let me rephrase my question since it seems to be way too confusing!

I have a spreadsheet with 4000 records that keeps on updating new data. The column headers are Employee Name, Project, Activity, 06/29/2013, 06/30/2013... etc. These are individual dates starting from October of last year to this week and each of these columns contains numeric values. I'm trying to modify this week-wise. So if Sunday is considered as the 7th day of a week, then I want to subtract the values from this week's Sunday with the last week's Sunday. And I'd want it to continue doing that for the rest of the columns since the dates will keep on updating so it's going to be taxing to do it manually.

FYI I'm using Excel 2013.
I know how to perform basic operations but this one is beyond my understanding so any help would be much appreciated!