# Birth time, time zones and apparent solar time

Why daylight saving, longitude and the late-Zi convention can change a chart, and how to make the calculation reproducible.

Canonical: https://wenbu.app/en/learn/birth-time-timezone/
Updated: 2026-09-30
Authorship: Wenbu · AI-assisted editorial

## The short answer

A recorded clock time identifies an instant only after its date and historical time zone are known. Solar time is a separate calculation convention, combining a longitude adjustment with the equation of time. Wenbu applies its approximate correction to the local clock used for day and hour pillars; year and month transitions still use the original instant.

- Resolve the recorded time zone before considering solar time. A solar-time switch cannot repair an incorrect date, DST assumption or UTC offset.
- Use positive longitude east of Greenwich and negative longitude west. Wenbu adds 4 × longitude − UTC offset in minutes + equation of time.
- Near a day or hour boundary, retain both sets of conventions and results. An approximate correction cannot remove uncertainty in the birth record.

**Check the recorded clock, the instant and the solar-time convention separately.**

Four cards identify the information needed to check the birth record, historical time zone, solar-time convention, and day/hour boundaries. Use them to locate the layer behind a disagreement. The full table below adds year/month transitions and common mistakes; the worked example then preserves the original instant alongside the correction.

## A clock time needs a place and a date

“Three in the afternoon” only identifies an instant when paired with the time zone and daylight-saving rules in effect on that date. Today’s UTC offset may differ from the historical offset at birth. Mainland China also observed daylight saving in some years.

Wenbu accepts IANA zones such as Asia/Shanghai and America/New_York. Do not substitute your current device zone for the zone where you were born, especially after moving countries.

## Some clock times are missing or repeated

A spring clock change can skip a time; an autumn change can repeat it. Wenbu’s BaZi calculator rejects these ambiguous or nonexistent inputs instead of silently picking an instant. Resolve the recorded time and supply an explicit UTC offset when necessary.

In New York, 02:30 on March 10, 2024 did not occur on the local civil clock. At the autumn transition, 01:30 on November 3, 2024 has two possible offsets. A birth record or other reliable historical information is needed to choose.

## Find the layer that differs between two charts

| Layer | Information to compare | A common mistake |
| --- | --- | --- |
| Birth record | Gregorian date, local clock time and precision | Entering a lunar date in a Gregorian field |
| Historical time zone | Birthplace IANA zone and offset on that date | Using the current residence or subtracting DST twice |
| Solar-time convention | On/off, longitude, correction and formula | Reversing longitude or confusing it with equation of time |
| Day and hour boundaries | Midnight or 23:00; late-Zi hour-stem convention | Comparing interpretation before checking different day pillars |
| Year and month transitions | The same instant and solar-term boundary | Applying the local solar correction to the transition again |

## What does solar-time correction change?

Local mean solar time follows longitude, at approximately four minutes per degree. Apparent solar time also accounts for the equation of time. Longitude, a time-zone meridian and daylight saving are different inputs.

Wenbu offers an approximate apparent-solar-time correction, requires a longitude and shows the correction in minutes. It is not a high-precision ephemeris. Near a pillar boundary, compare the corrected and uncorrected results rather than treating an approximate correction as definitive.

## Save the convention with the result

The default day boundary is midnight; a 23:00 Zi boundary is also available. In the library’s midnight convention, a late-Zi day pillar stays on the civil day while the hour stem advances. The chart discloses that choice.

Year and month pillars follow absolute solar-term boundaries. Day and hour pillars use the selected local clock. Exports preserve the settings so a discrepancy with another calculator can be investigated rather than explained away.

## Compare disagreeing charts one setting at a time

Check the Gregorian date and local clock time first, then the time zone and daylight-saving treatment. Next compare solar-term boundaries, the day boundary and solar-time correction. Change one setting at a time and note which pillar changes.

For example: “The year and month pillars agree, but the day pillar differs for a time near 23:00. Please compare the day-boundary rules first.” Use clearly labeled example details when you only need to demonstrate the issue.

## Work through a solar-time adjustment

Assume a record of December 23, 2005 at 08:37 in Asia/Shanghai and a teaching longitude of 121.5° east. This is a specified coordinate, not a claim that every Shanghai birthplace has that longitude. The equation of time uses Wenbu’s current approximate formula.

1. Resolve the historical offset: UTC+08:00, or +480 minutes. The original instant is 00:37 UTC. Keep that instant for year and month solar-term comparisons.

2. Calculate the longitude component: 121.5 × 4 − 480 = +6 minutes. The chosen point is 1.5° east of the 120° standard meridian, making local mean solar time six minutes ahead.

3. Add the formula’s equation-of-time value for the date, approximately +1.3 minutes. The total adjustment is about +7.3 minutes, producing approximately 08:44 apparent solar time. Displayed seconds do not turn the approximation into a precise observation.

4. Check the relevant boundaries. Both 08:37 and approximately 08:44 remain in the Chen hour and on the same day. The example therefore retains Yi You / Wu Zi / Xin Si / Ren Chen. Applying a correction does not necessarily change a pillar.

5. Save the original 08:37, the zone, 121.5° longitude, the enabled setting and the +7.3-minute correction. Do not enter 08:44 as the new original time with correction still enabled; that would apply the adjustment twice.

A reproducible chart preserves the recorded time and each transformation. The useful questions are whether a boundary was crossed and whether the birth record is precise enough, not how many decimal places the software displays.

## Key terms

**UTC offset** — The difference between a local clock and UTC on the relevant date; today’s offset is not necessarily the historical one.

**Local mean solar time** — Mean solar time expressed at a particular longitude, changing by about four minutes per degree.

**Equation of time** — Here, apparent minus mean solar time: a positive value means apparent solar time is ahead.

## Common questions

### Should an overseas birth be entered in Beijing time?

Enter the recorded local time and birthplace time zone. Wenbu internally uses fixed UTC+08:00 for the same instant when checking solar terms. Its day and hour calculation follows the selected local-clock convention. Converting the input yourself and retaining the birthplace zone would misidentify the instant.

### Should I subtract daylight saving after choosing an IANA zone?

Usually not: the time-zone library resolves the offset for the date. Wenbu rejects skipped or ambiguous local times during clock changes. Clarify the record and provide the intended offset rather than choosing one arbitrarily.

### Is apparent solar time always the correct choice?

It is a different convention with an astronomical definition. That definition does not establish which interpretive school makes more valid claims. State the convention first and compare results near a boundary.

## Sources and their scope

- [U.S. Naval Observatory · The Equation of Time](https://aa.usno.navy.mil/faq/eqtime) — Defines apparent time, mean time and their difference; example values come from Wenbu’s approximation, not an observatory ephemeris.
- [IANA · Time zone and daylight saving time data](https://data.iana.org/time-zones/tz-link.html) — Explains the location-based database of historical UTC offsets and daylight-saving rules.
- [Wenbu · BaZi calculation and declared conventions](https://github.com/Digidai/wenbu/blob/main/src/lib/bazi.ts) — Documents the formula, signs, rejection of ambiguous inputs and separate pillar handling; the +7.3-minute example was recalculated.
- [Hong Kong Observatory · The 24 Solar Terms](https://www.hko.gov.hk/en/gts/time/24solarterms.htm) — Explains solar terms as ecliptic-longitude boundaries, distinct from a local-clock or longitude adjustment.
- [Hong Kong Observatory · Gregorian–Lunar conversion tables](https://www.hko.gov.hk/en/gts/time/conversion.htm) — Further reading; not validation of real-world predictions.
- [lunar-typescript · calculation source and examples](https://github.com/6tail/lunar-typescript) — Further reading; not validation of real-world predictions.