Hi Mathi
This is very strange to me..
If your FY is Jan-Dec, then it should have posted 18 months of accum depreciation.. And you are saying your FY 2014 and 2015 is closed.. Was it closed when you did ABNAN first time?
Can you try in a test system? I am 100% confident it should be 18 months. Refer SAP Help, it says the same
Ajay M