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How to Enter Birth Time for BaZi: Local Clock Time vs. True Solar Time

Last updated: October 22, 2026

When casting a chart on Xiaodao, always enter the local civil clock time recorded at the birthplace (the exact hour and minute on the birth certificate or hospital record) and select the birth city from the location list.1 The calculator automatically adjusts for the city's geographic longitude, the daily Equation of Time, and historical Daylight Saving Time to compute True Solar Time. Do not manually convert a non-Chinese birth time to Beijing time or subtract longitude minutes yourself beforehand, or the adjustment will be applied twice.1

In traditional Four Pillars (BaZi) calculation, the Year Pillar and Month Pillar switch at the exact astronomical instant of a Solar Term (which does not vary by birthplace), while the Day Pillar and Hour Pillar are calculated from the birthplace's local True Solar Time (so birthplace longitude affects only the Day and Hour Pillars).2

Why Year and Month Follow Solar Terms While Day and Hour Follow Local Solar Time

A Four Pillars chart combines two distinct astronomical time measurements:23

  1. Year and Month Pillars (Astronomical Solar Longitude): The 24 Solar Terms mark Earth's position along its orbit around the Sun (for example, Li Chun occurs when the Sun reaches 315° ecliptic longitude). Because that orbital instant happens globally at the same moment, the Year Pillar (switching at Li Chun) and the Month Pillar (switching at the twelve major Jie solar terms) depend on the exact transition moment rather than birthplace longitude.
  2. Day and Hour Pillars (Local True Solar Time): The twelve traditional two-hour branches (shichen, from Zi around midnight to Wu around solar noon) track the Sun's daily arc across the local sky. Standard civil time zones assign a single clock time across wide geographic regions—most notably in mainland China, where the entire country uses UTC+8 anchored to the 120°E meridian. To determine which two-hour branch the Sun actually occupied at a specific birthplace, civil clock time must be converted into local True Solar Time.

How True Solar Time Is Calculated from Civil Clock Time

Converting recorded clock time to True Solar Time involves two steps:3

  • Step 1: Longitude Offset (Local Mean Solar Time). Earth rotates 360° in 24 hours, meaning each 1° of longitude equals 4 minutes of solar time (15° equals 1 hour). A city east of its time zone's reference meridian experiences solar noon earlier (+4 minutes per degree east); a city west of the reference meridian experiences solar noon later (-4 minutes per degree west).
  • Step 2: Equation of Time. Because Earth's orbit is elliptical and its axis is tilted, true solar noon drifts ahead of or behind mean solar noon by roughly -14 minutes to +16 minutes over the course of the year.

The table below shows how the same 12:00 PM civil clock time translates into different local mean solar times across several cities (before adding the date's specific Equation of Time):

Birth City ExampleCivil Time Zone & Reference MeridianApprox. LongitudeLongitude Difference from MeridianMean Solar Offset (Longitude Only)Approx. Local Solar Time for a 12:00 PM Clock Birth & Resulting Hour Branch
Shanghai, ChinaChina Standard Time (UTC+8, 120°E)~121.5°E~1.5° East+6 min~12:06 PM — falls squarely in the Wu (Horse) Hour (11:00–13:00)
Nanjing, ChinaChina Standard Time (UTC+8, 120°E)~118.8°E~1.2° West-5 min~11:55 AM — falls squarely in the Wu (Horse) Hour (11:00–13:00)
Chengdu, ChinaChina Standard Time (UTC+8, 120°E)~104.1°E~15.9° West-64 min (~1 hr 04 min behind)~10:56 AM — falls in the Si (Snake) Hour (09:00–11:00) on most days of the year, or crosses into Wu when the autumn Equation of Time is strongly positive
Urumqi, ChinaChina Standard Time (UTC+8, 120°E)~87.6°E~32.4° West-130 min (~2 hr 10 min behind)~09:50 AM — falls in the Si (Snake) Hour (09:00–11:00)
Kashgar, ChinaChina Standard Time (UTC+8, 120°E)~76.0°E~44.0° West-176 min (~2 hr 56 min behind)~09:04 AM — sits right near the boundary between the Chen (Dragon) (07:00–09:00) and Si (Snake) hours
New York, USAUS Eastern Time (UTC-5 at 75°W in winter; UTC-4 in summer DST)~74.0°W~1.0° East of 75°W+4 min in standard time (in summer, -60 min DST is removed first)~12:04 PM in winter (Wu Hour); ~11:04 AM in summer DST (near the Si / Wu boundary)
London, UKGMT / BST (UTC+0 at 0° in winter; UTC+1 in summer DST)~0.1°W~0.1° West of 0°~0 min in winter (in summer, -60 min BST is removed first)~12:00 PM in winter (Wu Hour); ~11:00 AM in summer BST (right at the Si / Wu boundary)

Three Practical Rules When Using Xiaodao's BaZi Calculator

When entering birth details in Cast a Bazi chart, follow these three rules to avoid double corrections:1

  1. Enter the exact local clock time recorded at birth: Whether the birth occurred in New York, London, Sydney, Shanghai, Chengdu, Urumqi, or Kashgar, enter the civil clock time used on the official record. Do not convert to Beijing time first, and do not manually subtract an hour for summer Daylight Saving Time (see Daylight Saving Time in BaZi).
  2. Select the specific birth city from the dropdown candidates: Choose "Elsewhere" or "China", type the city name (US, Canadian, and Australian cities include state or province names to prevent mix-ups), and click the matching candidate so the calculator can load its coordinates and historical time-zone rules.
  3. Confirm Zi-hour and boundary prompts when a birth falls near a transition: For births between 23:00 and 00:00 (11:00 PM to midnight local solar time), traditional BaZi has two conventions: switching the Day Pillar at 23:00 (23:00 starts the next day's Day Pillar) or splitting into Early and Late Zi (23:00–00:00 is Late Zi and keeps the current day's Day Pillar; 00:00–01:00 is Early Zi and takes the next day's Day Pillar). Xiaodao's input card includes an "Early/late Zi" setting (default "No", using the 23:00 day switch; "Yes" splits Early and Late Zi, with a note that non-specialists can leave it on the default; see BaZi 12 Two-Hour Branches Table & Early vs. Late Rat Hour). Whenever an adjusted solar time lands close to a two-hour branch transition, day boundary, or Solar Term transition, the page pops up a choice so you can confirm which side of the boundary to cast.

General-purpose chat models that generate BaZi charts without calling a dedicated astronomical calendar tool often make errors at boundaries such as Daylight Saving Time, True Solar Time longitude offsets, early/late Zi hour day changes, solar-term transition days, and Luck Pillar starting years; we recommend verifying any boundary chart with a calculator that applies full calendar rules.

Frequently Asked Questions

No. Converting your birth time to Beijing time before entering it is one of the most common mistakes in English BaZi calculators. Always select "Elsewhere", pick your actual birth city, and enter the local clock time recorded at your birthplace. Xiaodao handles both the astronomical solar-term comparison and local True Solar Time automatically.1

Cast a Chart with Automatic True Solar Time Adjustment

Enter a birth date, local clock time, and birth city in Xiaodao's free BaZi Calculator. Visitors can preview the four natal pillars and header details without signing in; signing up and saving a profile unlocks the Luck Pillars, annual, and monthly Timeline. First-time registration grants 200 Dao Points immediately upon sign-up. Or browse our free 11-chapter Sample Bazi Report.

Cast a Bazi chartSample report

Sources

  1. 1

    Xiaodao, public FAQ (https://xiaodaox.com/en/guide/faq), checked October 22, 2026.

  2. 2

    Wan Minying, San Ming Tong Hui, volume 2, “论四时节气” and “论年月日时.” Classical calendar structure for the Four Pillars.

  3. 3

    Hong Kong Observatory, Astronomical Time and Atomic Time and Solar Terms, checked October 22, 2026. Apparent solar time, mean solar time, and the astronomical definitions of the 24 Solar Terms.

  4. 4

    Xiaodao, public Privacy Policy (https://xiaodaox.com/en/privacy), checked October 22, 2026.

Learn

How to Enter Birth Time for BaZi: Local Clock Time vs. True Solar Time

Last updated: October 22, 2026

When casting a chart on Xiaodao, always enter the local civil clock time recorded at the birthplace (the exact hour and minute on the birth certificate or hospital record) and select the birth city from the location list.1 The calculator automatically adjusts for the city's geographic longitude, the daily Equation of Time, and historical Daylight Saving Time to compute True Solar Time. Do not manually convert a non-Chinese birth time to Beijing time or subtract longitude minutes yourself beforehand, or the adjustment will be applied twice.1

In traditional Four Pillars (BaZi) calculation, the Year Pillar and Month Pillar switch at the exact astronomical instant of a Solar Term (which does not vary by birthplace), while the Day Pillar and Hour Pillar are calculated from the birthplace's local True Solar Time (so birthplace longitude affects only the Day and Hour Pillars).2

Why Year and Month Follow Solar Terms While Day and Hour Follow Local Solar Time

A Four Pillars chart combines two distinct astronomical time measurements:23

  1. Year and Month Pillars (Astronomical Solar Longitude): The 24 Solar Terms mark Earth's position along its orbit around the Sun (for example, Li Chun occurs when the Sun reaches 315° ecliptic longitude). Because that orbital instant happens globally at the same moment, the Year Pillar (switching at Li Chun) and the Month Pillar (switching at the twelve major Jie solar terms) depend on the exact transition moment rather than birthplace longitude.
  2. Day and Hour Pillars (Local True Solar Time): The twelve traditional two-hour branches (shichen, from Zi around midnight to Wu around solar noon) track the Sun's daily arc across the local sky. Standard civil time zones assign a single clock time across wide geographic regions—most notably in mainland China, where the entire country uses UTC+8 anchored to the 120°E meridian. To determine which two-hour branch the Sun actually occupied at a specific birthplace, civil clock time must be converted into local True Solar Time.

How True Solar Time Is Calculated from Civil Clock Time

Converting recorded clock time to True Solar Time involves two steps:3

  • Step 1: Longitude Offset (Local Mean Solar Time). Earth rotates 360° in 24 hours, meaning each 1° of longitude equals 4 minutes of solar time (15° equals 1 hour). A city east of its time zone's reference meridian experiences solar noon earlier (+4 minutes per degree east); a city west of the reference meridian experiences solar noon later (-4 minutes per degree west).
  • Step 2: Equation of Time. Because Earth's orbit is elliptical and its axis is tilted, true solar noon drifts ahead of or behind mean solar noon by roughly -14 minutes to +16 minutes over the course of the year.

The table below shows how the same 12:00 PM civil clock time translates into different local mean solar times across several cities (before adding the date's specific Equation of Time):

Birth City ExampleCivil Time Zone & Reference MeridianApprox. LongitudeLongitude Difference from MeridianMean Solar Offset (Longitude Only)Approx. Local Solar Time for a 12:00 PM Clock Birth & Resulting Hour Branch
Shanghai, ChinaChina Standard Time (UTC+8, 120°E)~121.5°E~1.5° East+6 min~12:06 PM — falls squarely in the Wu (Horse) Hour (11:00–13:00)
Nanjing, ChinaChina Standard Time (UTC+8, 120°E)~118.8°E~1.2° West-5 min~11:55 AM — falls squarely in the Wu (Horse) Hour (11:00–13:00)
Chengdu, ChinaChina Standard Time (UTC+8, 120°E)~104.1°E~15.9° West-64 min (~1 hr 04 min behind)~10:56 AM — falls in the Si (Snake) Hour (09:00–11:00) on most days of the year, or crosses into Wu when the autumn Equation of Time is strongly positive
Urumqi, ChinaChina Standard Time (UTC+8, 120°E)~87.6°E~32.4° West-130 min (~2 hr 10 min behind)~09:50 AM — falls in the Si (Snake) Hour (09:00–11:00)
Kashgar, ChinaChina Standard Time (UTC+8, 120°E)~76.0°E~44.0° West-176 min (~2 hr 56 min behind)~09:04 AM — sits right near the boundary between the Chen (Dragon) (07:00–09:00) and Si (Snake) hours
New York, USAUS Eastern Time (UTC-5 at 75°W in winter; UTC-4 in summer DST)~74.0°W~1.0° East of 75°W+4 min in standard time (in summer, -60 min DST is removed first)~12:04 PM in winter (Wu Hour); ~11:04 AM in summer DST (near the Si / Wu boundary)
London, UKGMT / BST (UTC+0 at 0° in winter; UTC+1 in summer DST)~0.1°W~0.1° West of 0°~0 min in winter (in summer, -60 min BST is removed first)~12:00 PM in winter (Wu Hour); ~11:00 AM in summer BST (right at the Si / Wu boundary)

Three Practical Rules When Using Xiaodao's BaZi Calculator

When entering birth details in Cast a Bazi chart, follow these three rules to avoid double corrections:1

  1. Enter the exact local clock time recorded at birth: Whether the birth occurred in New York, London, Sydney, Shanghai, Chengdu, Urumqi, or Kashgar, enter the civil clock time used on the official record. Do not convert to Beijing time first, and do not manually subtract an hour for summer Daylight Saving Time (see Daylight Saving Time in BaZi).
  2. Select the specific birth city from the dropdown candidates: Choose "Elsewhere" or "China", type the city name (US, Canadian, and Australian cities include state or province names to prevent mix-ups), and click the matching candidate so the calculator can load its coordinates and historical time-zone rules.
  3. Confirm Zi-hour and boundary prompts when a birth falls near a transition: For births between 23:00 and 00:00 (11:00 PM to midnight local solar time), traditional BaZi has two conventions: switching the Day Pillar at 23:00 (23:00 starts the next day's Day Pillar) or splitting into Early and Late Zi (23:00–00:00 is Late Zi and keeps the current day's Day Pillar; 00:00–01:00 is Early Zi and takes the next day's Day Pillar). Xiaodao's input card includes an "Early/late Zi" setting (default "No", using the 23:00 day switch; "Yes" splits Early and Late Zi, with a note that non-specialists can leave it on the default; see BaZi 12 Two-Hour Branches Table & Early vs. Late Rat Hour). Whenever an adjusted solar time lands close to a two-hour branch transition, day boundary, or Solar Term transition, the page pops up a choice so you can confirm which side of the boundary to cast.

General-purpose chat models that generate BaZi charts without calling a dedicated astronomical calendar tool often make errors at boundaries such as Daylight Saving Time, True Solar Time longitude offsets, early/late Zi hour day changes, solar-term transition days, and Luck Pillar starting years; we recommend verifying any boundary chart with a calculator that applies full calendar rules.

Frequently Asked Questions

No. Converting your birth time to Beijing time before entering it is one of the most common mistakes in English BaZi calculators. Always select "Elsewhere", pick your actual birth city, and enter the local clock time recorded at your birthplace. Xiaodao handles both the astronomical solar-term comparison and local True Solar Time automatically.1

Cast a Chart with Automatic True Solar Time Adjustment

Enter a birth date, local clock time, and birth city in Xiaodao's free BaZi Calculator. Visitors can preview the four natal pillars and header details without signing in; signing up and saving a profile unlocks the Luck Pillars, annual, and monthly Timeline. First-time registration grants 200 Dao Points immediately upon sign-up. Or browse our free 11-chapter Sample Bazi Report.

Cast a Bazi chartSample report

Sources

  1. 1

    Xiaodao, public FAQ (https://xiaodaox.com/en/guide/faq), checked October 22, 2026.

  2. 2

    Wan Minying, San Ming Tong Hui, volume 2, “论四时节气” and “论年月日时.” Classical calendar structure for the Four Pillars.

  3. 3

    Hong Kong Observatory, Astronomical Time and Atomic Time and Solar Terms, checked October 22, 2026. Apparent solar time, mean solar time, and the astronomical definitions of the 24 Solar Terms.

  4. 4

    Xiaodao, public Privacy Policy (https://xiaodaox.com/en/privacy), checked October 22, 2026.