Time Zones, Local Clock Time, and Historical Sky Charts

“5:00 AM” is a civil-time statement. To reconstruct a sky, you still need to know what that clock reading meant at that place on that date.

Civil time is historical data

Timezone boundaries, standard offsets, and daylight-saving rules are set by governments and have changed repeatedly. The IANA tz database records these histories for representative locations and is updated as rules change.

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

Older dates deserve more caution

IANA explicitly notes that pre-1970 civil-time history is harder to represent comprehensively. For older records, a careful workflow may need local legal history, newspapers, almanacs, or institutional records in addition to a modern timezone database.

Sources: IANA — Theory and pragmatics of the tz database

Keep the recorded time and the resolved instant separate

The human record may say “5:00 AM.” The calculation needs an instant such as UTC. Preserving both lets you audit the conversion later instead of losing the original evidence.