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 true solar time instantly — and the answer that matters most: whether your hour pillar changes.

Correction result

Clock time

09:20

UTC+8 · 104.1°E

−63 min

True solar time

08:17

Where the sun actually is

Longitude correction: 104.1°E − 120.0°E = −15.93°; × 4 = −63.7 min
Local Mean Time LMT: 08:16; Clock + longitude
Equation of Time: +1 min; Varies by date, ±16 min
True solar time: 08:17; LMT + Equation of Time

巳

Clock double-hour
09:00–10:59

辰

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 — your hour pillar will differ, and the two charts read differently. Cast your chart with true solar time.

Cast chart with true solar time 2026 forecast Compatibility

True solar time = Local Mean Time (longitude correction) + Equation of Time (varies ±16 min by date). 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 measured by the sun’s actual position: the moment the sun reaches its highest point overhead is real noon for your location. Clock time is the administrative time shared across a whole time zone — the two can differ by tens of minutes, or over an hour.

In BaZi, the hour pillar is set by the birth double-hour, and that double-hour should follow the sun. For anyone born near the east or west edge of a time zone, the gap between clock time and true solar time is enough to shift the double-hour by one position — and once the hour pillar changes, the whole chart changes with it.

All of China runs on a single meridian (120°E, UTC+8) yet spans roughly 62 degrees of longitude. In Urumqi (87.6°E) true solar time runs about 130 minutes behind the clock — someone born at 10 a.m. there was really born before 8 by the sun.

True solar time = clock time + (local longitude − standard longitude) × 4 min + Equation of Time

The first part is Local Mean Time (LMT, longitude only); adding the day’s Equation of Time (EoT, which drifts within ±16 min by date) gives true (apparent) solar time.

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

Double-hour

Clock range

Rat

23:00–00:59

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

Enter your birth moment

Type the local clock date and time from your birth certificate or family memory.

Pick your birth city

Type a city name to auto-fill local longitude and the time-zone meridian, or enter longitude by hand.

See if the pillar shifts

A boundary crossing is highlighted, and one click carries the corrected details into a full chart.

Frequently asked questions

Does everyone need a true-solar-time correction?+

The further your birthplace sits from the time-zone meridian, the more it matters. Near the meridian the gap is only minutes; in far-western cities it reaches 1–2 hours, enough to change the double-hour. Anyone born within half an hour of a double-hour boundary should also check.

What is the difference between Local Mean Time (LMT) and true solar time?+

LMT applies the longitude correction only — clock time converted to the mean solar time at your birthplace. True solar time adds the Equation of Time on top: a date-dependent ±16-minute swing caused by Earth’s orbital eccentricity and axial tilt. This tool shows both values plus the EoT breakdown.

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: 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.