True Solar Time in BaZi: Should You Correct Your Birth Hour? (DST and the 23:00 Hour Explained)

True solar time is your birth time converted to where the sun actually was, using your birthplace's longitude and the equation of time. It can differ from the clock by a few minutes in some cities and by more than an hour in others. Because the BaZi hour pillar depends on which two-hour period you were born in, the question "should I correct to true solar time?" comes up constantly. This guide gives the formula, shows how far clocks drift from the sun in the cities our readers are born in, and settles daylight saving and the 23:00 hour.

The short version: if you were born more than half an hour from a two-hour boundary (the odd clock hours: 01:00, 03:00 … 23:00) and your birthplace sits near its time zone's standard meridian, the correction changes nothing. The people who need to look closely are those born near a boundary, those born far from the standard meridian, and anyone born under daylight saving time.

The formula

Clock time is shared by a whole time zone, but the sun does not care about zones. The earth turns one degree every four minutes, so the farther your birthplace is from the zone's standard meridian, the further the clock is from the sun.

True solar time = clock time + (birthplace longitude − standard meridian) × 4 minutes + equation of time

• Longitude offset: east of the standard meridian, the sun runs ahead of the clock, so add minutes; west of it, subtract.
• Equation of time: the earth's orbit is elliptical and its axis is tilted, so solar noon does not fall at the same clock time every day. The offset runs from about −14 minutes in mid-February to +16 minutes in early November, and is close to zero in mid-April, mid-June, early September and late December.

Add the two and you have the correction.

How far clocks drift from the sun

Two things stand out. Singapore and Malaysia run more than an hour ahead of the sun, because both adopted UTC+8 in 1982 for alignment with their neighbours even though they sit near 104°E. A correction there very often moves the hour pillar back by one double hour. Most North American and European cities, by contrast, are within half an hour of the sun, so the correction rarely matters.

When the hour pillar actually changes

The twelve double hours change at the odd clock hours. Only when the corrected time crosses one of those boundaries does the hour pillar change.

Births between roughly 22:30 and 01:30 deserve special care: a correction can move both the hour pillar and the day pillar, because the Zi hour straddles midnight (see below).

Subtract daylight saving time first

True solar time is computed from standard time. If the time on your birth record was daylight saving time, subtract one hour before applying any correction.

• United States and Canada: DST every year since 1967 in most states and provinces, with earlier periods in wartime and the 1970s energy crisis. Arizona (outside the Navajo Nation) and Hawaii do not observe it.
• United Kingdom and Europe: every year; the UK also ran double summer time during the Second World War.
• Australia: most south-eastern states every year; Queensland and Western Australia do not.
• Singapore and Malaysia: no DST, but both used UTC+7:30 until the end of 1981. A 1975 birth at 10:00 in Singapore was recorded in UTC+7:30, so its clock was only 35 minutes ahead of the sun, not 65.
• Taiwan: intermittently between 1945 and 1979. Mainland China: 1986 to 1991. Japan: 1948 to 1951. South Korea: 1987 and 1988, and earlier spells in the 1950s.

The two common mistakes are forgetting to subtract it, and subtracting it by hand when the calculator already does, which removes two hours. GuanXiang's calculator applies the historical time zone and DST rules of the birthplace region automatically: enter the time exactly as written on the birth certificate and do not adjust it yourself.

The 23:00 hour: which day does a late-night birth belong to?

The Zi hour runs from 23:00 to 01:00 and is the only double hour that crosses midnight. For births between 23:00 and 00:00, the "late Zi hour", schools differ on which day pillar applies:

The hour branch is Zi under both conventions, but the day stem can differ, which changes the hour stem and every Ten God relationship in the chart. GuanXiang changes the day at 23:00 by default. If you were born in this window, cast both versions and compare; the Zi hour guide sets out the rules side by side.

If a true solar time correction pushes your time across 23:00, both the hour and the day pillar move, and the two charts can look very different. Look at both.

So should you correct?

This is a difference between schools, not a right answer. Those who correct argue that the hour pillar was always meant to follow the sun. Those who do not point out that practitioners have historically used the official local time, and that minute-level birth records are a modern luxury.

A practical approach:

1. Cast the chart with the time on your birth certificate. It is the most reliable record you have.
2. Check your distance from the nearest two-hour boundary and from your zone's standard meridian. Born in New York at 10:00? Use the chart as is.
3. Born near a boundary, or in Singapore, Malaysia or another far-from-meridian place? Cast both versions, see which pillar differs, and compare both against what you know of yourself.
4. Never adjust the birth time to "reach" a pillar you prefer. Changing the time changes the chart; it is not a correction.

GuanXiang's BaZi calculator uses the birthplace's local clock time by default. When you know the birth city, you can switch on the true solar time correction in the form; the result page shows how many minutes were applied and whether the hour pillar changed.

Frequently Asked Questions

What is the difference between mean solar time and true solar time?

Mean solar time corrects only for longitude and assumes the sun reaches due south at the same clock time every day. True solar time adds the equation of time, which tracks where the sun really is. The two differ by up to 16 minutes. When a calculator says "true solar time correction", it usually applies both.

Do people born in Singapore or Malaysia always need the correction?

Not always, but more often than anywhere else our readers come from. Clocks there run more than an hour ahead of the sun, so the corrected time frequently falls in the previous double hour. Cast both charts and compare. Births before 1982 were recorded in UTC+7:30, which shrinks the gap to about half an hour.

I don't know my birth city. What should I do?

Then do not correct. True solar time needs a longitude, and without a reliable birthplace there is no reliable correction. Use the time on your birth record.

The correction changed my hour pillar. Which chart is right?

The casting itself cannot prove it. You can compare how the two charts differ against your known life and temperament, but that is a judgement aid, not evidence. Keeping both charts with their conventions noted is more honest than forcing a choice.

Read next

• Zi hour BaZi: how to read charts for births near midnight or with an unknown birth time
• How to calculate a BaZi chart by hand: year, month, day and hour pillars
• Stems and Branches calculator: four pillars and the Chinese double-hour table
• How to read a BaZi chart: a practical beginner's guide