When does the day change?
子時 begins at 23:00 and runs past midnight, so a birth after 23:00 takes the next day’s pillar. That is the BaZi convention, and it is not the same as the civil calendar’s midnight rollover.
TODAY’S 8 CHARACTERS · HOW IT IS CALCULATED
True Solar Time
True Solar Time is where the sun actually stood at your birthplace, which is not the same as the clock on the wall. The gap can exceed an hour — Singapore’s clocks run about 65 minutes ahead of its local sun — and that is enough to move the Hour pillar into the neighbouring 時辰, or to flip the Day pillar across the 子時 boundary. So the first step of casting a chart is converting the birth time, not looking it up.
Clock time vs solar time
Standard time is an administrative convenience: one meridian serves a whole time zone. True Solar Time follows the sun instead, four minutes earlier for every degree east. BaZi uses the second, because solar terms and 時辰 are solar concepts to begin with.
| Aspect | Clock time | True Solar Time |
|---|---|---|
| Based on | The time zone’s standard meridian | The sun’s actual position locally |
| Same zone, different longitude | Identical | 4 minutes per degree |
| Daylight saving | Clocks jump an hour | Unaffected |
| Used for | Daily life, travel, paperwork | Chart casting, 時辰, sundials |
The two corrections
Two corrections stand between a clock reading and True Solar Time. The first is longitude: every degree between your birthplace and the zone’s standard meridian is four minutes. The second is the equation of time — an elliptical orbit on a tilted axis makes the sun cross the meridian up to sixteen minutes early or late through the year.
Undo daylight saving first
If daylight saving was in force at the birthplace, the clock was pushed forward an hour; subtract it before anything else.
Then correct for longitude
Longitude of birth − longitude of the zone’s standard meridian, times four minutes. Hong Kong (114.17°E) sits 5.83° west of the 120°E meridian, so its sun runs about 23 minutes behind the clock.
Finally add the equation of time
Look up (or compute) that date’s equation of time — roughly between −14 and +16 minutes.
Worked examples
The table shows the longitude correction alone (the equation of time adds up to another sixteen minutes). A positive number means the local sun runs ahead of the clock.
| City | Longitude | vs clock time |
|---|---|---|
| Singapore | 103.82°E | ≈ 65 min behind |
| Guangzhou | 113.26°E | ≈ 27 min behind |
| Hong Kong | 114.17°E | ≈ 23 min behind |
| Beijing | 116.41°E | ≈ 14 min behind |
| Shanghai | 121.47°E | ≈ 6 min ahead |
| London (summer) | 0.13°W | ≈ 60 min behind |
Why it moves the chart
The Hour pillar changes every two hours — 子時 runs 23:00–01:00, 丑時 01:00–03:00, and so on. A birth on or near a boundary (23:0x, or a few minutes before 01:00) can shift pillars on the strength of that conversion alone.
Not just the Hour pillar. The Day pillar rolls over when 子時 begins at 23:00, not at midnight — so someone born just after eleven at night gets the *next* day’s pillar. It is easy to miss, and it decides the Day Master.
How this app handles it
Pick the birth city and the app applies the corrections above — the city’s longitude, its zone and its daylight-saving setting — before handing the moment to the calendar engine. Both the longitude offset and the equation of time are included, and the conversion happens here: birth data is never sent anywhere.
If the hour is unknown, say so. The Hour pillar is shown as a noon representative value, clearly labelled as an assumption, and hour-derived readings are skipped rather than dressed up as if noon were real.
Common questions
Why is Singapore so far off?
Singapore sits at about 103.8°E but keeps UTC+8, whose standard meridian is 120°E — an hour and a bit east of where it actually is, so its clocks run about 65 minutes ahead of its own sun. It is exactly why the birthplace, not the time zone, drives the conversion.
What if I don’t know my birth time?
Ask family, or check a birth certificate first. If nothing turns up, mark the hour as unknown: the year, month and day pillars stand, and only the hour-derived parts are skipped.