If we're being really pedantic, in the U.S. context the zones observe "standard time" all year. "Standard time" refers to the standardization across the zone, and the practice of advancing the clock during daylight-saving time changes each zone's standard time. The Unix-style usage of "EST" vs. "EDT" isn't pedantically correct (e.g., New York observes "eastern standard time" even in summer).
See 15 U.S.C. §§ 260a & 263 (https://www.law.cornell.edu/uscode/text/15/chapter-6/subchap...).