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...).
> and the practice of advancing the clock during daylight-saving time changes each zone's standard time.
Even more pedantically, “standard time” is not necessarily consistent across each zone (particularly, during the period for which in parts of the zone it is advanced by an hour) since "standard time” only advances for those states, or parts of states, for which an exemption is not in place.
So, the Unix-y convention of using PST for "Pacific Standard Time without advancement", and PDT for "Pacific Standard Time with advancement" is the simplest way of getting meaningful concise labels out of the US legal scheme. (This is only a theoretical issue for some US timezones, but it is a concrete one for at least the Pacific and Hawaii-Aleutian Time Zones.)
> Even more pedantically, “standard time” is not necessarily consistent across each zone (particularly, during the period for which in parts of the zone it is advanced by an hour) since "standard time” only advances for those states, or parts of states, for which an exemption is not in place.
I can't find a source (including 15 U.S.C. § 260a) that supports this reading, although I agree it's a little ambiguous. The law suggests that a region that doesn't observe DST is observing "the standard time otherwise applicable during that period" and is exempt from the provisions regarding advancement, not that "Pacific standard time" depends on where you are (see 15 U.S.C. § 263).
> So, the Unix-y convention [] is the simplest way
No argument there!
> I can't find a source (including 15 U.S.C. § 260a) that supports this reading
From 15 USD § 260a, right after laying out the baseline rule for advancing standard time: “... however, (1) any State that lies entirely within one time zone may by law exempt itself from the provisions of this subsection providing for the advancement of time, but only if that law provides that the entire State (including all political subdivisions thereof) shall observe the standard time otherwise applicable during that period, and (2) any State with parts thereof in more than one time zone may by law exempt either the entire State as provided in (1) or may exempt the entire area of the State lying within any time zone.”
There are places that are exempt from the legal rule advancing standard time, hence, in those places, standard time does not advance.