Currently, the twelve-month period beginning October 1 and ending September 30. Historically, until 1831 and from 1843-49, the twelve-month period ending September 30 of the respective year; from 1832-42 and 1850-67, ending December 31 of the respective year; from 1868-1976, ending June 30 of the respective year. The transition quarter (TQ) for 1976 covers the three-month period, July-September 1976.
Source: U.S. Citizenship and Immigration Services
For Medicare, a year-long period that runs from October 1st through September 30th of the next year. The government and some insurance companies follow a budget that is planned for a fiscal year.
The budget year for the United States Government. It begins on October 1 and ends on September 30 of the following year.
Source: U.S. Department of State.