These questions would probably be a better fit in the The Marketing World/Advertising Sales and Affiliate Programs forum category.
The 7 day EPC when applied to the advertisers, shows the average EPC (earnings per 100 clicks) for ALL of the publishers using their links for the last 7 days. The 30 day EPC shows results for the last 30 days.
In your individual publisher reports, the 7 day EPC is an estimate of what you will earn for 100 clicks based on YOUR results for the last 7 days.
(7 day earnings/number of clicks)*(100)
You might also want to read the FAQ which is found in the support section on CJ's site.