Ottawa Corporation owns machinery that cost $20,000 when purchased on July 1, 2014. Depreciation has been recorded at a rate of $2,400 per year, resulting in a balance in accumulated depreciation of $8,400 at December 31, 2017. The machinery is sold on September 1, 2018, for $10,500. Prepare journal entries to (a) update depreciation for 2018 and (b) record the sale.

My book got depreciation expense = 2400 * 8/12= 1600 , but how is it 8 months? but I am getting 10 months when I count dec 31 to sept 1 INCLUSIVE? Am i supposed to count dec 31 to sept 1 inclusive ? because I am getting 10 months not 8. I know the solution to the question, but I just don't see how it is 8 months... could someone help please???

thanks in advance