Civil Time Is Historical Data

Astronomy asks for an instant. Human records usually give you a clock reading. The bridge between them is historical civil time.

Time zones are not natural boundaries

They are legal and administrative systems. Their offsets, borders, abbreviations, and daylight-saving rules can change, sometimes with little notice.

Sources: IANA — Time zone and daylight saving time data

The further back you go, the messier it gets

IANA notes significant limitations in comprehensive pre-1970 coverage because local practices before modern computerized timekeeping are difficult to reconstruct globally.

Sources: IANA — Theory and pragmatics of the tz database

Preserve the transformation

A good record keeps the original local timestamp, the timezone interpretation, the resulting UTC instant, and the source or database version used. That makes a later correction possible without erasing the original evidence.