Hi Ram
I will keep it simple...
1. You need to specify when the depreciation was last posted in Legacy system (OAYC)
2. Based on 1, system comes to know from when it has to post.... Once you do (1) as 31.03.2012, system knows it has to calculate dep from Apr 2012
3. Now, you are doing this activity in Dec.. So, you can run the depreciation (AFAB) each month starting from Apr using PLANNED posting option
OR
you can run with UNPLANNED posting option, directly for Dec month, which will post dep for 9 months into the current month
So, the calculation will happen from Apr onwards, but the posting can happen in respective months or cumulatively in Current month
Hope this helps
Br, Ajay M