Daylight Saving Time Can Change a Historical Birth-Sky Reconstruction

A local clock reading is incomplete until you know which offset was in force at that place on that date.

Rules change by jurisdiction and year

Timezone and daylight-saving rules are political decisions. The IANA tz database records many historical transitions, including years when the U.S. changed the dates on which daylight time began or ended.

Sources: IANA — How to Read the tz Database · IANA — Time zone and daylight saving time data

Ambiguous and skipped local times exist

When clocks move backward, some wall-clock times occur twice; when they move forward, some local times do not occur at all. A historical reconstruction should preserve the original local time and record how that ambiguity was resolved.

Sources: IANA — Theory and pragmatics of the tz database

Do not infer from the modern rule

A person born decades ago may have been subject to a different national, state, or local rule. Resolve the timezone from the historical date and location, not from what the same place does today.