26 CFR §301.7701(b)-6 — (b)-6 Taxable year.

1. (a)In general. An alien individual who has not established a fiscal year as his or her taxable year prior to the period that the individual is subject to United States income tax as a resident or a nonresident shall adopt the calendar year as his or her taxable year. An alien who has established a fiscal year in a foreign country prior to the period that the individual is subject to United States income tax may adopt the calendar year as his or her taxable year for United States income tax purposes without requesting a change in accounting period. An individual will be considered to have established a fiscal year (whether in the United States or a foreign country) if the annual accounting period on which the individual computes his or her income is a fiscal year, the individual keeps his or her books in accordance with that fiscal year, and the requirements of section 441 and § 1.441-1(b) of this chapter are otherwise satisfied. An alien who has established a fiscal year and is a resident alien during the calendar year will be treated as a resident alien with respect to any portion of his or her taxable year (beginning with the individual's residency starting date and ending with the individual's residency termination date) that falls within such calendar year. Once the individual has established either a fiscal or calendar year taxable year for any period for which the individual is subject to United States income tax, the individual may not change that taxable year without the approval of the Secretary. See section 442.
2. (b)Examples. The following examples illustrate the operation of this section: