Free tool
True Solar Time Calculator
Enter your birthplace and original clock time to compare solar dates, double-hours and boundaries before casting a chart.
Example · Chengdu 09:20
This demonstrates Chengdu on 1992-04-18. It is not your result. Fill in your details to preview your calculation.
True solar time · Apparent solar time
08:17
Clock time: 1992-04-18 09:20 · UTC+08:00
Difference from clock time: −63.0 min
Clock double-hour: Snake → Dragon
Crosses a double-hour boundary
The corrected date is unchanged.
What each of these times means
- Longitude and legal-time correction
- 104.07° × 4 − 8 × 60 → −63.7 min
- Local Mean Time
- 08:16
- Equation of Time
- +0.8 min
This page and its chart link explicitly use apparent solar time: longitude correction plus the date’s Equation of Time. Other chart entries may have different defaults; check the selected mode.
The Equation of Time uses a low-order approximation and the display rounds to minutes; this is not zero-error astronomy. The uncertainty range uses the selected UTC offset. Check each side separately if the record spans a DST change.
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. The meridian used here is derived from the effective UTC offset at birth, including DST.
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 + longitude×4 − UTC offset at birth×60 + Equation of Time (minutes)
Step one (longitude and the historical legal offset) 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?+
This page and its chart link explicitly use apparent solar time: longitude correction plus the date’s Equation of Time. Other chart entries may have different defaults; check the selected mode.
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?+
Enter the original clock time without subtracting DST. The chosen IANA zone resolves the offset at the birth date and time. Missing or repeated wall times require checking the record. If the zone is unavailable, enter the historical UTC offset including DST. Pre-1970 records may be incomplete.