BaZi Birth Time Correction: The Time on Your Certificate Is Not Always the Time Your Chart Uses
In BaZi the hour pillar changes every two hours, and the month pillar changes at the exact moment of a solar term. A birth certificate, though, records clock time. In daylight-saving years the clock ran a full hour fast, and a time zone keeps one clock for a whole region, whether you were born at its eastern edge or a thousand miles west. Correcting for these gaps gets you back to the sun's time, which is what a BaZi chart is really built on.
Four things to check before you cast a chart
| Correction | How big | What it can change | Who should look twice |
|---|---|---|---|
| Daylight saving time | A full 60 minutes | The hour pillar, occasionally the day pillar | Anyone born under DST: in Taiwan, summers 1946–1979 and all of 1937–1945; in Hong Kong, the US, UK, Europe and Australia, many summers |
| True solar time | In Taiwan about −21 to +23 minutes | The hour pillar | Anyone born near an odd hour (1, 3, 5…); far more for western China, Singapore and Malaysia |
| Zi-hour day change | The 23:00–23:59 hour | The day pillar (the Day Master) | Anyone born after 11 p.m. |
| Solar term boundary | The moment the term changes | The month pillar, and at the Start of Spring the year pillar too | Anyone born around the 4th to 8th of a month, on the day a term changes |
How we counted: our engine drew random birth times (to the minute) within each period, 12,000 per group, and compared the four pillars cast straight from the clock hour with the exact, corrected pillars. The modern Taiwan group was born 1980–2010; the daylight-saving group, 1946–1979.
Taiwan sits close to the 120°E standard meridian, so its longitude offset is only a few minutes, and for recent births the equation of time does most of the work: about 6.8% change hour. The daylight-saving era is a different story. Among Taipei births on DST days, about 43.0% change hour once corrected; across all of 1946–1979 it is 16.9%. Hong Kong ran DST for longer in the same years and lies further west, so 46.6% change hour. Singapore's clocks run half an hour to over an hour ahead of the sun, and 29.4% change.
Daylight saving time: a Si hour on the clock that was really Chen
Born June 10, 1975, 9:10 a.m., Taipei
- Daylight saving: from April 1 to September 30, 1975, Taiwan's clocks ran an hour fast. The birth certificate says 9:10; standard time was 08:10.
- True solar time: Taipei sits at 121.56°E, a degree and a half east of the zone's standard meridian, so add 6 minutes; the equation of time on June 10 is about +1 minutes. True solar time comes to 08:17.
- Result: entered as the Si hour straight from the clock, the hour pillar is Yi-Si (乙巳). Corrected, the birth falls in the Chen hour and the hour pillar is Jia-Chen (甲辰). The year, month and day pillars stay the same, but the whole hour pillar changes.
Correct your birth time
Enter the Gregorian date, time and place from the birth certificate. The tool works through daylight saving time, the longitude offset, the equation of time, the Zi-hour day change and the solar term boundary in one go, then compares the result pillar by pillar with a chart cast straight from the clock hour. Everything is calculated in your browser; nothing is uploaded.
Enter the clock time exactly as the birth certificate shows it; the tool takes off daylight saving time itself, so do not subtract it first. The day changes at 11 p.m. If your birthplace is not listed, pick the city closest in longitude (one degree is only four minutes).
How this site handles it
- The chart form on our home page asks for the date and the two-hour period only, with no true solar time correction. Most people remember clock time, and schools already disagree on whether to correct at all.
- The in-depth reading takes off daylight saving time and converts to true solar time automatically when you tick the exact-time option and give a birthplace.
- The tool on this page lays out every layer of the correction and uses the exact moment of the solar term to set the year and month pillars, which covers the blind spot of forms that only take the hour (see solar term boundaries).
Three common misunderstandings
The time on my birth certificate is my BaZi time.
A birth certificate records the clock time of the day. Taiwan alone ran daylight saving time in 19 summers between 1946 and 1979, and used Japan time all year from 1937 to 1945; on all those days the clock ran an hour ahead of standard time. Most countries with DST have a similar history.
True solar time must be applied, or the chart is wrong.
That is a choice between schools. The classical Zi Ping method uses local solar time, but many teachers today use clock time, arguing that birth times carry their own error. We calculate both and let you compare.
Born after Chinese New Year means the new year has started.
A BaZi year changes at the Start of Spring (Li Chun), not on the first day of the lunar year. If you were born between late January and early February, check the exact moment of the Start of Spring; see solar term boundaries.
Read next
A straight word: correction only helps if the recorded time is accurate
Hospital records usually give the minute, but family memory is more often "a bit after nine in the morning." If the time could be ten or twenty minutes out, fine-tuning it is pointless. If your time is an estimate, rather than worrying about whether to correct it, cast both possible hours and see which chart sounds more like you. Everything on this site is for reference and entertainment.
Time corrected? Cast the full chart
Tick the exact-time option and give your birthplace, and the in-depth reading makes the same corrections before writing up twelve facets of your chart: Ten Gods, strength, favorable elements and luck pillars.
Birth time correction FAQ
Should I use the time on my birth certificate for BaZi?
That time is clock time. If you were born during daylight saving time, subtract an hour first. The more careful approach then converts it to true solar time using the birthplace's longitude and the equation of time.
Which years did Taiwan use daylight saving time?
Taiwan used daylight saving time in 1946, 1947, 1948, 1949, 1950, 1951, 1952, 1953, 1954, 1955, 1956, 1957, 1958, 1959, 1960, 1961, 1974, 1975, 1979. From October 1937 to September 1945 it also ran on Japan time, which put the clock an hour ahead of today's. The start and end dates for each year are on the daylight saving time page.
Do I need true solar time if I was born in Taiwan?
Taiwan's longitude offset is only a few minutes either way; the bigger effect is the equation of time, which ranges from about −14.6 to +16.5 minutes over the year. In our sample about 6.8% of recent Taiwan births change hour because of it, mostly those born close to an odd hour.
Born at 11 p.m.: which day is my day pillar?
We change the day at 23:00, so the day pillar belongs to the next day. Another school changes the day only at midnight. The two give different Day Masters; the Early and Late Zi hour page explains both.
What if I don't know the minute I was born?
If you only know the rough two-hour period, correction adds little. Cast both likely hours instead and compare which chart fits your life better.