It varies from business to business and industry to industry. Businesses use what is known as a "fiscal" year for accounting purposes. This can be the same as the calender year, but many times it is not.
Usually, a business will have it's fiscal year end during an off season in order to use the slow down as the opportunity to do inventory and all the other financial stuff that goes in to wrapping it up. Thus, for example, most retail businesses do not use a calender year because it comes on the heels of Christmas sales.
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