Accounting periods include calendar years and fiscal years, during which accounting functions are performed, analyzed, and aggregated.
A company may be operating in more than one accounting period at any given time. The financial records of an entity may be closed for the month of June, for example. This means the accounting period is the month (June), although the entity may also wish to aggregate its accounting data into quarters (April through June), halves (January through June), and an entire calendar year.
An accounting period only pertains to the income statement and cash flow statement since the balance sheet shows information as of the date of the statement. For example, if an entity reports on its results for January, the income statement header states "for the month ended January 31," while the balance sheet header states "as of January 31."
In investing, the accounting period is useful because potential shareholders can analyze a company's performance by examining its financial statements.