Condition in time category

[spikyman]

Hi Guys!
Can anyone help me with this job :

I need to add another date option called Trailing Months which would be the prior 12 months up through the last quarter.  So
if it were April 2013, I would see Mar 2012 to Mar 2013.  If it were May 2013, I would see April 2012 to March 2013.

I did it in sql server and I use it in relational package in our manual Relative dates dimension. But I need add this new time period to Time dimension in cube.
This time period starts with current month -13 and finish with last month of prior quarter.
So when is feb 2013 the it starts on jan 2012 and ends with dec 2012
When will be march 2013, it will start on feb 2012 and it will end on dec 2012...
I found that there is the same logic for first months of current quarter(jan,apr,jul,oct), second months(feb,may,aug,nov) and third months(mar,jun,sep,dec).
I dont want to have three new time categories but only one.
Is it possible to somehow check in transformer which month is current month and then supress or add some months to this time category?
I attach image where you can see how it looks with three time categories.
Thank you very much for your time and help.

With regards Vladimir.