Quarter.
Quarter. A 3-month period beginning January 1, April 1, July 1, or October 1.
Source
27 CFR § 17.11
Scoping language
As used in this part, unless the context otherwise requires, terms have the meanings given in this section. Words in the plural form include the singular, and vice versa, and words indicating the masculine gender include the feminine. The terms includes and including do not exclude things not listed which are in the same general class.