- Local civil time: what the birth record says.
- Standard time: civil time after any daylight-saving adjustment is removed.
- True solar time: a derived value using longitude and equation of time.
1. What local clock time records
Civil time lets a region coordinate daily life under a legal time zone. Everyone in a wide zone may share the same clock even though their longitudes differ. The birth record should remain preserved because it is the original evidence.
2. What true solar time estimates
True solar time estimates the apparent position of the Sun at the birthplace. The derivation has two main parts.
Longitude correction
Compare the city's longitude with the legal time zone's standard meridian, at roughly four minutes per degree.
Equation of time
Add the date-dependent difference caused by Earth's orbital shape and axial tilt.
3. Why the difference can be large
Singapore uses UTC+8 although its longitude lies far west of the UTC+8 reference meridian. A 12:00 civil reading can therefore produce a true solar value more than an hour earlier. This may change the hour branch.
4. Where methods differ
Some schools prioritize civil or standard time; others use true solar time. Midnight and Zi-hour rules can add another difference. The rigorous response is method disclosure and candidate comparison, not declaring every other chart wrong.