Free tool
True Solar Time Calculator
The clock time on your birth certificate isn’t the sun’s real time. Enter your birth moment and city to get Local Mean Time and true solar time — and the answer that matters most: whether your hour pillar changes.
Correction result
Clock time
09:20
UTC+8 · 104.1°E
Local mean time LMT
08:16
Longitude correction only (auxiliary)
True solar time
08:17
The sun’s true position in your birth sky
- Longitude correction
- 104.1°E − 120.0°E = −15.93°
- × 4 = −63.7 min
- Local Mean Time LMT
- 08:16
- Clock time + longitude
- Equation of Time
- +1 min
- Varies by date, ±16 min
- True solar time
- 08:17
- LMT + Equation of Time
What each of these times means
- Clock time (zone standard time)
- The time recorded on your birth certificate — the legal time of your time zone. Astronomically it equals the mean solar time of the zone’s standard meridian: regions on UTC+8, for example, all keep 120°E time, regardless of where the sun actually stood at your birthplace.
- Local Mean Time (LMT)
- Clock time corrected for longitude: 4 minutes earlier per degree west of the meridian, 4 minutes later per degree east. It restores “one shared zone time” to the mean solar time above your birthplace — but assumes the sun moves at a uniform pace all year.
- Equation of Time (EoT)
- The gap between the real sun and the uniform “mean sun”: Earth’s elliptical orbit and axial tilt make true solar days uneven across the year, so the two differ by up to ±16 minutes depending on the date.
- True solar time
- Local Mean Time plus the Equation of Time — the time that matches the sun’s actual position in your birth sky. This is what traditional astrology means by “casting with true solar time.”
Which time does BaZiFlow cast charts with?
Charts are cast with clock time by default; Local Mean Time takes over automatically only when the birthplace sits far from its time-zone meridian, and the Equation of Time is never applied to charts. The threshold: most regions need a longitude gap of 30+ minutes; for places like mainland China, where the whole country keeps a single UTC+8 time, the line is 97.5°E (a gap of roughly 90+ minutes). The reasoning: the Equation of Time swings only ±16 minutes across the year — rarely enough to move a double-hour, and mostly noise — while feedback across a large body of real charts shows this “clock time + large-gap longitude correction” convention casts noticeably more accurate charts. The true solar time above is shown for reference; your actual chart follows the site’s default convention.
Clock double-hour
09:00–10:59
True solar double-hour
07:00–08:59
Crosses a double-hour boundary — Clock time falls in the Snake hour, but true solar time falls in the Dragon hour — the two conventions give different hour pillars and completely different hour-pillar readings. Carry both into a chart and compare.
True solar time = clock time + longitude correction (4 min per degree) + Equation of Time (varies by date within ±16 min). Local Mean Time (LMT) is the intermediate, longitude-only value, shown as an auxiliary reference. Historical daylight saving isn’t included: for births during China’s 1986–1991 summer-time months, subtract 1 hour before entering.
What is true solar time?
True solar time is the time that matches the sun’s actual position in the sky above your birthplace. It comes from clock time in two steps: first the longitude difference against the time-zone meridian gives Local Mean Time (LMT); then the date-dependent Equation of Time (EoT) is added on top.
Each degree away from the standard meridian moves Local Mean Time by 4 minutes — Chengdu (about 104°E) runs roughly 64 minutes behind Beijing time. The Equation of Time comes from Earth’s elliptical orbit and axial tilt, swinging within ±16 minutes across the year. Clock time itself is, astronomically, the mean solar time of the standard meridian.
In BaZi, the hour pillar is set by the birth double-hour. When clock time sits near a double-hour boundary, or the birthplace lies far from the meridian, true solar time can shift the double-hour by one position — and once the hour pillar changes, the whole chart changes with it. That is why all of these times are worth checking before casting a chart.
True solar time = clock time + (local longitude − standard longitude) × 4 min + Equation of Time
Step one (longitude) produces Local Mean Time, shown here as an auxiliary intermediate value; step two, the Equation of Time, swings within ±16 minutes depending on the birth date — adding it yields true solar time.
The twelve double-hours
| Double-hour | Clock range |
|---|---|
| Rat | 23:00–00:59 |
| Ox | 01:00–02:59 |
| Tiger | 03:00–04:59 |
| Rabbit | 05:00–06:59 |
| Dragon | 07:00–08:59 |
| Snake | 09:00–10:59 |
| Horse | 11:00–12:59 |
| Goat | 13:00–14:59 |
| Monkey | 15:00–16:59 |
| Rooster | 17:00–18:59 |
| Dog | 19:00–20:59 |
| Pig | 21:00–22:59 |
How to use this true solar time calculator
01
Enter your birth moment
Type the local clock date and time from your birth certificate or family memory.
02
Pick your birth city
Type a city name to auto-fill local longitude and the time-zone meridian, or enter longitude by hand.
03
See if the pillar shifts
A boundary crossing is highlighted, and one click carries the corrected details into a full chart.
Frequently asked questions
Which time does BaZiFlow cast charts with by default?+
Charts are cast with clock time by default; Local Mean Time takes over automatically only when the birthplace sits far from its time-zone meridian, and the Equation of Time is never applied to charts. The threshold: most regions need a longitude gap of 30+ minutes; for places like mainland China, where the whole country keeps a single UTC+8 time, the line is 97.5°E (a gap of roughly 90+ minutes). Two reasons: the Equation of Time swings only ±16 minutes across the year — rarely enough to move a double-hour, mostly noise — and more importantly, feedback across a large body of real charts shows this “clock time + large-gap longitude correction” convention casts noticeably more accurate charts. The true solar time shown here is for reference and comparison; your actual chart follows the default convention.
What is the difference between Local Mean Time (LMT) and true solar time?+
LMT applies only the first step — the longitude correction — restoring “one national time” to the mean solar time above your birthplace. True solar time adds the second step, the Equation of Time: a date-dependent ±16-minute swing caused by Earth’s orbital eccentricity and axial tilt. This tool shows both: LMT as the auxiliary intermediate value, true solar time as the final result.
If the double-hour changes, does the chart differ a lot?+
A different hour pillar changes the hour-branch Ten Gods, the children palace and later-life timing, and can shift the overall structure. That is exactly why this tool exists: get the times clear and confirm the hour first, then cast the chart.
What about daylight saving time?+
China observed summer time (clocks forward 1 hour) in the summers of 1986–1991. If you were born during one of those daylight-saving months, subtract 1 hour before entering. Full historical time-zone handling is planned for a later version.