One welcome addition to OBIEE 11g Answers has been the ability to do time series analysis. This was only possible using the repository admin tool in 10g.
The Formula Editor now has the ability to do AGO, TODATE and Rolling Period. The big problem is that the UI does not make it very easy. In order to do these calculations you must provide a time_level property in the function. But, unlike the admin tool, it does not provide anywhere to select the time dimension or any levels.
I looked into the documentation, and if it is there I could not find it so I resorted to hitting my head against the wall a few times. As it turns out if you know the name of the time dimension and the hierarchy and level you can manually enter it.
If you are using the SampleApp the Time dimension is "Time" and the Hierarchy is "Time Hierarchy" and the Level is "Year". So putting this all together the time_level = "Time"."Time Hierarchy"."Year"
So for a new measure called Rev YTD it would look something like this:
And the resulting Results would look like this:
Not hard at all once you figure it out!
Hope this helps!