A birth record gives the civil clock time used at the birthplace. True solar time adjusts that reading toward the Sun’s apparent position at that longitude. The difference is often measured in tens of minutes and can change the Hour Pillar in western China, countries spanning wide time zones, or births near a two-hour boundary.
Step one: recover historical local time
The calculation cannot apply today’s time-zone rules to a birth decades ago. It must look up the UTC offset active for that city and date, including historical daylight-saving changes. Only after the local clock record is tied to a UTC instant can solar correction begin.
At the start of daylight saving, some local times never occurred; at the end, some occurred twice. A system must disclose that ambiguity rather than silently guess.
Step two: longitude and equation of time
A time zone normally refers to a standard meridian. Each degree of longitude away from it corresponds to about four minutes of local solar difference. Earth’s orbit and axial tilt add the equation of time, which changes through the year.
This site first derives the standard meridian from the historical offset, then applies longitude difference and the equation of time. The program performs the calculation; a language model does not estimate it, and Chinese birth times are not subjected to a blanket one-hour subtraction.
When the correction changes the chart
If clock and corrected time remain within the same two-hour branch interval, the Hour Branch normally stays unchanged. Crossing a Zi, Chou, Yin or other boundary changes the Hour Pillar. Near the 23:00 rollover, true solar time may also move the Day Pillar. Births near Start of Spring or a monthly Jie boundary require checks of the Year and Month Pillars as well.
True solar time is this site’s fixed convention. Other practitioners may retain civil time or use another day rollover. Those differences can be explained, but users are not invited to switch methods until a more attractive chart appears.