Event Studio has a different purpose from Report Studio. It is designed to allow you to detect conditions occuring and then fire off one or more tasks (reports, emails, other events, stored procedures, etc etc).
Each event centres around a condition being true or false. The condition is defined as an expression, and can be as simple or as complex as you need it to be (as long as the end result is true or false), so yes - it is entirely possible to write an expression which compares a measure value from six weeks ago with a current value (depending on your package, of course).
Regards,
MF.