BAZI KNOWLEDGE BASE
Historical time zones: which offset belongs to a birth record?
A birth record saying “10 a.m.” identifies an instant only when paired with the clock rules used at that place and date. A city may have used summer time or changed its standard offset. The zone shown on a phone today cannot establish what a clock meant decades ago.
The same 10 a.m. reading in two Hong Kong summers
The Hong Kong Observatory records summer time from 13 May to 21 October 1979 and none in 1980. On 1 July, safely away from either transition, 10:00 in Hong Kong therefore converts from UTC+9 to 01:00 UTC in 1979, and from UTC+8 to 02:00 UTC in 1980. These are examples of each year’s conversion, not the same birth instant.
Read the sign and minutes of a UTC offset
Subtract the signed UTC offset from local time. If a record explicitly uses +05:45, 10:00 becomes 04:15 UTC; at −05:00, 10:00 becomes 15:00 UTC. The value +05:45 is not 5.45 decimal hours. A midnight crossing also changes the date, and an unspecified offset does not mean zero.
An offset describes one conversion. It cannot identify the city or its full clock history. Keep the date, region and resulting offset together rather than reducing the record to a numbered zone.
Why an IANA name helps more than an abbreviation
Names such as Asia/Hong_Kong and America/New_York identify regional clock histories. Abbreviations such as CST and IST have multiple meanings; three letters are insufficient to choose a region. An older name may also be an alias for the same history.
Usually, begin with the city and country or territory, then check the selected zone. The city in an IANA name is a representative location, not a guarantee of equally complete records for every historical settlement.
How much precision does the historical record support?
IANA cautions that pre-1970 information does not cover all places completely. Some old offsets contain seconds, but a database entry with seconds does not mean every chart tool preserves them. Clarify currently handles offsets in whole minutes, so complete historical second-level support should not be assumed.
Documenting the date, original time, source and chosen zone is more useful than adding unsupported seconds. Correcting an offset may leave the pillars unchanged if no relevant hour, day or term boundary is crossed. Find the changed pillar before discussing its interpretation.