Custom Measure for Last Year

Hi,

To simplify report creation for the users and avoid member offset, I'd like to create one measure for Last Year YTD and one for Last Year Periodic (i.e. if I create a report where Time = 2014.May and Measures = YTD, Periodic, LYYTD, LYPeriodic, I'd like to see 2014.May YTD and Periodic, and then 2013.May YTD and Periodic). I'm working in a YTD model in BPC10MS. I've found the LAG function but I don't know what the syntax would look like. Anybody having an idea?

/Cecilia