Kairo
Korean Saju
Patterns before predictions
How Time Zones and DST Affect a Saju Chart
A birthplace isn't a decorative field on the form. It's what tells the calculator what the recorded clock actually meant.
A Saju birth time must be interpreted within the legal clock system used at the birthplace on that date. Historical time-zone offsets and daylight saving shift the recorded instant by an hour or more, changing the Hour Pillar and, near a day boundary, the Day Pillar too. Longitude-based solar-time correction is a separate convention and should be disclosed.
The city tells the calculator what the recorded clock actually meant. It isn't a decorative field on the form.
Recorded time is not a universal time
"8:30 a.m." describes a local clock reading. To compare it with solar terms or convert it reliably, software needs:
- city and country
- date
- historical UTC offset
- whether daylight saving was active
- any exceptional legal-time change
Using the current time zone for a birth decades ago can be wrong.
The IANA Time Zone Database records many historical changes and is a common technical source for software. It has known gaps for some historical localities. Even with those gaps, it beats manually assigning a present-day offset to a decades-old birth record.
How daylight saving changes the chart
During daylight saving, clocks are moved—often one hour ahead of standard time. If a calculator ignores that rule, it may place the birth in the wrong traditional two-hour period.
Example:
- recorded birth: 3:20 a.m. during daylight saving
- standard-time equivalent: 2:20 a.m.
If a traditional Hour Branch boundary lies between those times under the chosen convention, the Hour Pillar changes.
The example illustrates the mechanism, not a universal branch assignment. Exact results depend on method and location.
Time-zone conversion versus solar-time correction
These are different operations.
Time-zone conversion determines the actual instant represented by the legal local clock.
Solar-time correction adjusts clock time according to longitude and, in some methods, the equation of time, to approximate the Sun's local position.
Some Saju practitioners use recorded standard time. Others apply longitude correction or true solar time. A service should not describe its choice as the only traditional method when schools differ.
Should an overseas birth be converted to Korea time?
Not as a simple replacement.
A person born in Chicago at 9:00 a.m. was born at a specific global instant in Chicago's local context. The calculator may convert that instant internally, but it should not pretend the birth occurred at the corresponding Seoul clock time and discard location.
The relevant solar and calendar calculations should be performed consistently under the service's disclosed method.
Why births near boundaries deserve special treatment
Time corrections matter most when the recorded time is close to:
- an Hour Branch boundary
- midnight or Rat-hour day boundary
- a solar-term transition
- a legal clock change
If the birth sits far from every relevant boundary, a one-hour dispute does not change the visible Four Pillars. It can still affect finer timing calculations, so the service should distinguish material differences from immaterial ones.
What if the place name is ambiguous?
"Springfield," "Cambridge" or "Seoul area" may not be precise enough for automated geocoding.
Use city, state/province and country. Historical borders and renamed cities can also confuse software. The goal is to recover latitude, longitude and time-zone history—not to standardise the birthplace into the nearest famous city.
For hospital births outside a town centre, minute-level location precision is rarely the biggest uncertainty unless the time is close to a boundary. Do not manufacture geographic precision while ignoring a rounded birth time.
A transparent time calculation note
A report can state:
Birth data used: 14 March 1992, 09:20, Chicago, Illinois, USA. Historical local time-zone rules were applied. Kairo's stated solar-time convention was then used. The result does not cross an Hour Pillar boundary.
Or, for uncertainty:
The recorded time is within 15 minutes of an Hour Pillar boundary after correction. Hour-dependent interpretation is therefore presented as conditional.
These notes make accuracy auditable.
Questions to ask an online service
- Do you use a historical time-zone database?
- Is daylight saving detected automatically?
- Do you use recorded time or solar-time correction?
- How do you handle locations outside Korea?
- What happens near an hour or day boundary?
- Will the report show the final pillars?
If the answer is "our AI handles it," ask for the actual rule.
Precision should follow the weakest input
A calculator may know the solar-term instant to the second. If the family remembers only "around breakfast," the final chart is not second-level precise.
Good methodology preserves the precision of each source instead of letting sophisticated software disguise uncertain input.