Running asset depreciation for few months

Dear gurus,

now we have some assets that we must sell/dimiss soon. We must run the depreciation before selling/dismissing but we think that by doing this, the final depreciation will be calculated for one year, and not from 01/01/2015 to 30/09/2015.

We think we should create new custom keys and assign them to assets but our doubt is about how to create a custom key which calculate the depreciation from 01/01/2015 to  30/09/2015?

Do you have any idea?

Thanks for your support

Gianni