Time zones and daylight saving

To compute a chart, the local birth time is converted to Universal Time (UTC). An offset that's off by one hour moves the Ascendant by roughly 15° and reshuffles the houses, so the time zone deserves a minute of your attention.

How the offset is found

Picking a city fills “Time zone” with its IANA zone. The offset for the birth date comes from the IANA tz database, including the daylight saving rules of that year. The info line above the wheel shows the result, for example UTC+7 (time zone Asia/Barnaul) → 1982-09-20 22:57:00 UT.

Regional history corrections

The IANA database knows capital cities well and the history of individual regions of the former USSR and Eastern Europe much less so. For dates before 1993, Astrodamus checks the city against its own table of historical zones. If a different time was in force there that year, the chart uses it, the info line says “corrected from regional history”, and a yellow warning explains what changed and how reliable it is (“documented” or “probable”).

Warnings

Warning	What to do
Time zone corrected…	Compare with the birth certificate. If it says otherwise, set the offset by hand
Offset uncertain…	Sources disagree. Choose the value that matches the certificate and enter it in “UTC offset (manual)”
Before 1970, IANA time zone data can be inaccurate…	Check the offset against a reference or the certificate
The time … did not exist (clocks moved forward for DST)	Clocks skipped that time on that day. Double-check the birth record
The time … occurred twice (clocks moved back)	That hour repeated. If you need the other one, set the offset by hand

Set the offset by hand

Find the difference from Greenwich at the moment of birth, for example +4 or −3.5.
Type it into “UTC offset (manual)”. The 0.25-hour step covers zones like +5:45.
Click “Build chart”. The info line now says “(manual)”.

A manual offset applies to the birth moment only. Transits, calendars and forecasts for present-day dates still use the zone in “Time zone”.