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Is it ethical for a company to assume assets are in use on the first day of the following month after they are purchased when computing depreciation? For example, if assets were purchased Feb. 8, but they are recorded in use starting March 1. I know the previous rule for depreciation assumes all assets are in use on the first day of the month closest to the purchase date. I didn't know if it was okay to do it the following month or not? Also, if assets were assumed in use on the following month, would this increase the profit margin?

Not necessary. Company can adapt any method as far as using the correct data.
Look, LIFO, and FIFO make a difference on net profit and taxable income. Doesn't mean LIFO is not allowed because it gives you chance to get taxed less.

What do you mean as fara as using the correct data? So, are you saying that it is okay to use the depreciation rule that asset additions are to be in use the first day of the following month and if so how does this affect the profit margin of a business?