Apparent solar time: why a birth chart may adjust your clock time

What should I enter?

Enter the date and clock time written in the local birth record, together with the birthplace. Do not convert it to Taiwan time, subtract daylight saving or apply a solar correction first. The free BaZi calculator resolves those steps from the location and historical time zone. No account is needed to calculate a chart.

Read the original and corrected dates and times in the result. Ask two separate questions: did the time cross an odd-hour boundary such as 23:00, and did it cross midnight? If the birth time is unknown, keep it unknown; a noon placeholder cannot establish a precise correction.

Two corrections, with different purposes

Civil time uses a time zone shared by a region. Local mean solar time instead follows the birthplace’s longitude. The Earth turns about one degree every four minutes, so a place east of a zone’s reference meridian is ahead of that zone’s mean solar time.

Apparent solar time then adds the equation of time: the seasonal difference between a uniformly running clock and the apparent motion of the Sun. It varies through the year because the Earth’s orbit and axial tilt are not a uniform sundial.

For a fixed standard-time offset, the calculation can be expressed as: apparent solar time = standard clock time + 4 × (longitude − reference meridian), in minutes, + equation of time. The reference meridian is 15 degrees times the standard UTC offset in hours. Daylight-saving time must first be removed when applicable.

Near the international date line, that expression also needs a civil-date convention. MingLiMR anchors the calculation to the birthplace’s civil date and selects the nearest solar day to the local clock, so a positive or negative longitude does not by itself shift the result by a whole day. In minutes, the product’s correction is 4 × longitude − the actual UTC offset + 1440 × date-line days + equation of time. The date-line days are the nearest integer to (the actual UTC offset − 4 × longitude) / 1440; the actual offset includes any daylight-saving time in effect. Enter the recorded local clock time in the tool without adjusting it yourself.

For a fixed-offset example, longitude −171.75° and UTC+13:00 give −1467 minutes before the equation of time. Adding one date-line day changes that to −27 minutes before the equation of time. This civil-date anchoring is the site’s calendar convention, separate from the astronomical approximation used for the equation of time.

A worked clock example

Use the fictional time 2025-11-15 22:50 in Taipei, Asia/Taipei, longitude 121.56° E. The historical offset is UTC+8 without daylight saving on that date. The worked table below uses the same time-resolution function as chart calculation, including the equation of time for that exact date.

The positive correction moves this example past 23:00, from Hai to Zi, while leaving it on the same date. Crossing an hour boundary and crossing midnight are different events. Recalculate it in the BaZi calculator, or inspect the corresponding palace calculation in the Zi Wei calculator, using the original 22:50 clock reading. Do not enter the already corrected result as the birth time.

Taipei is east of the 120° E reference meridian associated with UTC+8. Its longitude correction adds several minutes; the equation of time can then subtract or add more. The familiar approximate range of minus ten to plus twenty-two minutes describes Taipei, not every place on Earth.

A clock time close to 01:00, 03:00 or another two-hour boundary can consequently fall into a different traditional hour. A time near midnight can cross into a different calendar day. Do not adjust the hour while silently keeping the old date.

Taiwan city and monthly tables

The reference tables below use the same city longitudes and equation-of-time function as the existing Chinese article. The city column is the longitude component only; the monthly column is a dated illustration of the seasonal component. Adding a city number to a monthly sample gives an estimate, not the exact correction for every day in that month.

City-center coordinates also cannot recover an uncertain birth record. If you only know “around dawn,” a correction reported to the minute does not turn that memory into a precise time.

Births outside Taiwan

Use the actual birthplace and the historical civil time zone. A present-day UTC offset is not necessarily the offset used on the birth date. Daylight-saving rules, changes to standard time and repeated or skipped clock times can all matter.

The application resolves the civil instant and applies the solar-time convention as distinct steps. An ambiguous clock time during a fall-back transition needs a choice of which occurrence is intended. A nonexistent time during a spring-forward gap should not be silently accepted as an ordinary time.

The solar-term boundary is an astronomical instant. Compare the resolved birth instant with it; do not assume that everyone born on the calendar date of a solar term is already on the new side.

Date rollover and the Zi hour

If 23:50 becomes 00:10 on the following day after correction, the chart must use the following date. If 00:10 becomes 23:50 on the previous day, it must move back. MingLiMR’s split Zi-hour convention then applies to that corrected date and time. Read the Zi-hour explanation for the separate school difference at 23:00.

An earlier implementation corrected the displayed time without carrying the date. That was a calculation defect rather than a school preference. The corrected behavior is covered by regression checks.

What to check when charts differ

Compare the original civil date and time, birthplace, historical zone, daylight-saving treatment, solar-time setting and day-boundary convention. A difference in any one can change the hour pillar; a rollover can change the day pillar and Day Master as well.

Saved charts retain the data and calculations associated with their version; changing interface language is not a request to recalculate an old chart. If birth time is unknown, the site cannot determine solar-time rollover. It uses a provisional civil-date three-pillar chart where possible and asks for time on an ambiguous solar-term date.