True solar time: longitude, equation of time, and DST
Quick answer: True solar time (真太阳时) means the hour pillar follows the Sun at your birthplace, not necessarily the time on the civil clock. On this site we (1) convert local clock time to a UTC instant with the IANA time-zone database, (2) remove daylight saving when it applied, (3) apply a longitude correction of about four minutes per degree from the zone meridian, and (4) add the equation of time. You can turn true solar time off and use clock time instead. The receipt shows every minute.
The technical specification is methodology § From your clock to the birth instant. This page is the plain-language ask: why does my hour differ from another calculator?
Civil time is not the Sun
Modern clocks keep mean time inside a time zone. A zone can stretch many degrees of longitude, so people on the eastern and western edges share one clock while local solar noon differs. Daylight saving shifts the clock again without moving the Sun. BaZi hour branches are two-hour solar houses starting at Zi (23:00–01:00). If you feed the engine daylight-saving clock time as if it were solar time, births near a double-hour edge can land in the wrong hour pillar.
That is the whole motive for true solar time: align the hour pillar with apparent solar time at the birthplace.
What we do, in order
1. Time zone → UTC
We interpret the birth clock using the IANA database for the selected place, including historical daylight-saving rules. Ambiguous clock times (repeated when clocks fall back) use the earlier occurrence and say so. Nonexistent times (sprung forward) are flagged.
2. Remove daylight saving (default on)
Daylight saving moves the clock, not the Sun. For the hour pillar we use the zone's standard time, which means taking the extra hour back out of a DST birth. The receipt shows how many minutes were removed.
3. Longitude correction
Standard zones are centred on a meridian (for example UTC+8 near 120°E). If you were born west or east of that meridian, local mean solar time differs by roughly 4 minutes per degree of longitude.
4. Equation of time
Even after longitude, apparent solar time drifts from mean solar time through the year (Earth's orbit and axial tilt). NOAA's published approximation places that difference roughly in the −14 to +16 minute range across the year. We follow NOAA's form:
time offset = equation of time + 4 × longitude − 60 × time zone
(with the signs and units as in the methodology write-up and NOAA's solar-position notes).
5. Optional switch
True solar time is on by default when you give a birthplace. You can turn it off and use clock time (still with DST handled per our defaults unless you also change other switches). Without a birthplace, we cannot apply longitude/equation-of-time the same way; see limitations on methodology.
When TST actually changes the chart
True solar time only changes pillars when the correction pushes the birth across a boundary:
| Boundary | What can flip |
|---|---|
| Double-hour edge (every 2 hours from 23:00) | Hour pillar |
| 23:00 day change (our default) | Day pillar and Day Master; hour stem reckoning |
| Midnight (if you switched day boundary) | Day pillar under that school |
| Solar-term edge (rare for hour-scale corrections) | Month pillar; at Li Chun, year too |
Most births are far from edges; TST then changes the receipt more than the pillars. Births within about 10 minutes of a double-hour edge get a warning that the hour depends on the exact minute — see methodology § hour.
Day-boundary education: Does the BaZi day start at 23:00 or midnight?. Year-boundary education: Li Chun.
Worked intuition (not a second engine)
Suppose a city sits well west of its zone meridian. Longitude correction can subtract a double-digit number of minutes. In February the equation of time also tends to be negative in NOAA's curve. Stacked on a DST removal of 60 minutes in summer, the solar time used for the hour can differ from the hospital clock by more than an hour in the extreme — enough to move Zi ↔ Chou or to cross 23:00.
We always print the minutes on the receipt so you can add them by hand.
Why two calculators disagree
| Cause | What to check |
|---|---|
| One uses clock time, one uses TST | Our switch + receipt minutes |
| One leaves DST in | Our receipt "DST removed" line |
| Different coordinates for the same city name | Lon/lat on the receipt |
| Day boundary 23:00 vs midnight | Zi-hour page |
| Year at Chinese New Year vs Li Chun | Li Chun page — unrelated to TST, but often blamed on "time" |
Methodology's disagreement table is the checklist: where schools disagree.
True solar time and missing birth times
If you do not know the birth hour, TST cannot create one. We show three pillars and no hour — we never fill in noon. See Can I cast a BaZi chart without a birth time?. Day Master usually still stands; only births near 23:00/midnight make the day stem convention-sensitive even without a precise minute.
Sources of the astronomy (not classical verses)
True solar time here is astronomy applied to the hour pillar, not a quotation from Di Tian Sui. Classical practice assumes solar double-hours; modern civil time needs the corrections above. We cite NOAA's solar-position equations on the methodology Sources list. Classical book pages still matter for stems, gods and luck — see Classical sources — but they are not the formula for equation of time.
How to verify on your chart
- Open Find your chart with place enabled.
- Note clock time vs true solar time on the receipt.
- Toggle TST off and compare hour pillars.
- If you were born near a double-hour edge, read the boundary warning.
- If only the Day Master matters for a first pass, use the Day Master calculator and return here before trusting an hour-sensitive read.
For the Learn reading order, see Learn BaZi.
Longitude in everyday terms
Time zones are wide. Inside UTC+8 alone, a birth in western China and a birth on the coast can share a civil clock while local solar noon differs by an hour or more. The four-minutes-per-degree rule is the mean-solar piece of that gap. Equation of time then nudges apparent solar time away from mean solar time through the seasons.
You do not need to memorise NOAA's curve. You need to read the minutes printed on the receipt and ask whether those minutes cross a double-hour line.
DST myths
- "Summer births should keep the daylight-saving hour because the Sun feels later." Feeling is not the civil database. We remove the legal DST offset so the solar correction starts from standard time.
- "My hospital wrote BST/PDT, so the engine is wrong to subtract." The hospital recorded civil time correctly; the engine then translates civil → solar for BaZi hours. The receipt should show the subtraction explicitly.
- "Countries without DST can ignore this page." Longitude and equation of time still apply.
Comparing with a printed Chinese almanac
Traditional almanacs assume solar double-hours in a China-centred workflow. Diaspora births need zone databases and coordinates. If an almanac chart matches our natal pillars after you enter Beijing for a Beijing birth, that does not prove a Toronto birth with clock time will match without TST. Always prefer the receipt over screenshots without settings.
Accessibility of switches
TST on/off and day-boundary midnight/23:00 are independent. Power users reconciling teachers from different schools should change one switch at a time and note the pillar diff. Methodology's disagreement table is the map; this page is the TST column of that map.
Do I need true solar time?
If your birth is far from hour and day edges, pillars may match clock time anyway. If you are comparing with another site, match TST and DST settings before concluding the engines differ.
Does TST change the year pillar?
Only in the unusual case where corrections move the instant across Li Chun (or another month jie). Ordinary longitude/EoT minutes usually do not. Year/month flips are most often about the civil date sitting near Li Chun, not about TST.
Is true solar time the same as "solar time apps"?
Many astronomy apps show local apparent solar time. Our receipt should be close if coordinates and DST handling match. We follow the NOAA-style offset documented on methodology.
What about births before 1970?
The time-zone database is less complete for some places before 1970; charts carry a note. If a birth certificate gives an exact UTC offset, compare it with the receipt.
Can I leave TST on and still switch the day boundary?
Yes. TST and the 23:00/midnight choice are separate controls. Read both pages if you are near late night.
Equation of time without the integral calculus
Apparent solar time runs fast or slow relative to mean solar time during the year. February is typically among the "Sun runs slow" stretches in NOAA's approximation; other months run fast. Stacked on longitude, that is why two births at the same civil clock in the same zone but different months can show different TST offsets on the receipt. BaZi care is simple: trust the printed offset; watch double-hour edges.
Birthplace pickers and large metro areas
Choosing "Tokyo" vs a suburban hospital can move longitude by a fraction of a degree (about 4 minutes per degree). That rarely flips pillars by itself, but combined with a birth already 3 minutes from an hour edge, it can. If you know the hospital, prefer the more precise place entry when the UI allows.
Next step: Calculate your chart with a birthplace, open the receipt, and read the longitude and equation-of-time minutes yourself.
Sources
- Clock → UTC → DST → TST pipeline: How we calculate your chart
- NOAA equation-of-time reference (linked from methodology Sources)
- Related boundaries: Li Chun · Zi-hour day boundary
- Editorial context: About
BaZi is a traditional Chinese cultural practice. This page explains a time convention; it is not advice about medical induction times or legal records.
Last updated: 2026-10-07