The PREVMEMBER functionality will give you the previous member on the same hierarchy level. 1-JAN-2011 ==> 31-DEC-2010, 2-JAN-2011 ==> 1-JAN-2011, CY 2011 ==> CY 2010.
The NEXTMEMBER functionality will give you the next member on the same hierarchy level. 1-JAN-2011 ==> 2-JAN-2011, 31-DEC-2011 ==> 1-JAN-2012, CY 2011 ==> CY2012
We can use this calculate difference between periods.