How we calculated it

This guide describes the Pillars phone app. The website does the same calculations, but some screens and features work differently or aren't here yet.

A good cook can tell you the recipe step by step. This screen does the same with your chart: it shows each step the app took, with your own numbers, from the birth certificate to the four pillars.

To open it, go to the Chart tab and tap How we calculated it, or tap the convention chip under your name.

Calculated on this device

The first card says it plainly: everything is calculated on your phone, offline, with no servers. The time zone tables and the ephemeris, the tables of where the sun is, come built into the app.

The steps

Birth record: the date, time and city you entered, and how sure the time is.
Instant in UTC: the birth moment in world time. The app looks up the offset your city really used that day, including daylight saving time as it was back then, from the IANA time zone tables.
True solar time: the time the sun showed in your city. It corrects for your longitude and for the equation of time, a small seasonal difference between the clock and the sun.
Position of the sun: how many degrees the sun had travelled along its path. That decides your BaZi year and month. If the sun hadn't yet reached 315°, the Start of Spring (立春), the screen tells you the year still counts as the previous one.
Day and hour: your Day pillar and the branch of your hour, plus how many minutes you were from the nearest hour boundary. A small number means the hour was close to changing.
Conventions: the rules used, like true solar time and the day starting at 11 pm, years from Lì Chūn and months by solar terms.

If the birth time isn't exact, the app can't show one single instant. It works out every possible time window and marks what changes.

Compare conventions

Near the end, a card called Compare conventions checks the same birth with clock time instead of sun time. In many places the pillars come out the same. Where clocks drift far from the sun, the Hour pillar can change, and the card tells you how.

The calculation ID

At the very bottom there's a Calculation ID, a code like calc- followed by sixteen letters and numbers. It works like a fingerprint. It's made from the birth details, the conventions and the versions of the tables and rules the app used. Same inputs, same ID.

You'll see the same ID on every page of the PDF report and in Settings. If a printed reading and the app show the same ID, they come from the same calculation.

What's under the hood

In Settings, under About the calculation, you'll find the technical names: the sun's position from VSOP87D, solar terms from 1900 to 2100 built into the app, and the IANA time zone tables. The results are cross-checked against an independent lunar calendar library and published charts.

A chart made from imported pillars has no calculation to show. Those pillars are shown just as they were typed.