Case 14 of 26
True-solar crossings
London: Equation of Time crosses an hour boundary
Near the February Equation-of-Time minimum, a small clock difference can change the hour branch.
Boundary question
Does Equation of Time participate even near the reference meridian?
Yes. The 01:10 record corrects to 00:55, crossing back into Zi Hour despite longitude being close to Greenwich.
Synthetic input
Recorded facts
- Date
- 1990-02-12
- Recorded time
- 01:10
- Location
- London, United Kingdom
- Time zone
- Europe/London
- Longitude
- -0.1276Β°
- Gender input
- Female
Pinned expected facts
Verified output
Year
εΊε
Month
ζε―
Day
ζη³
Hour
壬ε
- Day Master
- ζ
- True solar time
- 1990-02-12 00:55
- Total correction
- -14.73 minutes
- Historical offset
- UTC+00:00 Β· DST 0 min
- Luck direction
- ιζ
- Reference Jie
- η«ζ₯ Β· 1990-02-04 10:14:00
- Luck start
- 1992-08-25 11:27:16
Reading the boundary
What this case means for your own chart
London sits almost on the Greenwich meridian, so the longitude correction is nearly zero. A reader might reasonably assume that a London record needs no adjustment at all. This case shows the Equation of Time doing the work by itself: 01:10 corrects back to 00:55, crossing into Zi Hour on longitude alone being irrelevant.
The Equation of Time is the difference between apparent solar time and mean solar time, and it swings by roughly a quarter of an hour over the year. In mid-February it is near its annual minimum. Fifteen minutes is small in absolute terms, and decisive when a record sits a few minutes on the wrong side of a boundary.
For readers, the lesson is that no location is exempt from the correction. Being near the reference meridian removes one term from the sum, not the sum itself. The remaining terms are small, but small is not the same as negligible near a boundary.
It is also the term most likely to be missing from a simpler calculator, because it cannot be derived from a timezone database. Longitude comes from the birth place and the offset comes from historical records; both are lookups. The Equation of Time is a formula, and a tool that does not implement it will look correct everywhere except close to a boundary.
Check your own record
- Do not skip the solar-time correction on the assumption that your birth place is close to its reference meridian. The Equation of Time still applies.
- Estimate how far your birth time sits from the nearest two-hour boundary before deciding the correction cannot matter.
- For records within about fifteen minutes of a boundary, treat the Equation of Time as potentially decisive.
Which convention is in play
- Calculation fact
- The pinned engine returns 00:55 true solar time for this 01:10 London input, with a correction of about β15 minutes dominated by the Equation of Time rather than longitude.
- Convention
- Whether the Equation of Time belongs in the correction pipeline at all is a point on which practitioners differ. This engine includes it and states the formula source.
- Our explanation
- The framing that no location is exempt is our own emphasis, chosen because proximity to Greenwich is a common reason readers assume no correction is needed.
Related reading
Conventions and limits
What this case does β and does not β prove
It verifies
Deterministic reproduction of this input under the named engine version and conventions. A pinned-field mismatch stops the production build.
It does not claim
Universal agreement across Bazi schools, interpretation of a real person, or certainty where the historical record is incomplete.
tyme4ts 1.5.2 Β· cantian-tymext 0.0.27 Β· sect 2 day boundary Β· BaziCalculator.ai NOAA/IANA true-solar adapter
calculation-benchmarks-v1 Β· reviewed 2026-09-05