True Solar Time

Why Standard Clock Time Falls Short

Time zones span up to 15 degrees of longitude. Locations on the eastern and western edges of the same zone can differ by up to an hour in actual solar position. In China, a single time zone (UTC+8) covers the entire country, a region spanning five natural time zones. Someone born in far western China at noon clock time might have a true solar time of around 10 AM. That difference is large enough to shift them into a different birth hour period entirely.

The Two Corrections

True Solar Time applies two adjustments. The first is longitude correction, which compensates for the difference between your birth city's longitude and the time zone's reference meridian. Each degree of longitude equals 4 minutes of time. The second is the equation of time, a seasonal correction for variations in Earth's orbital speed and axial tilt. It can add or subtract up to 16 minutes depending on the date.

Impact on Chart Accuracy

In both BaZi and Ziwei Doushu, the birth hour determines a key component. In BaZi, it defines the Hour Pillar, one of the four Heavenly Stem and Earthly Branch pairs. In Ziwei Doushu, it determines which palace becomes the Life Palace and how all other palaces align. Chinese metaphysics divides the day into twelve two-hour periods, so even a 30-minute correction can push you across a period boundary. That alone could change your Day Master's Hour Pillar or your entire Ziwei Doushu palace configuration.

City Longitude Reference Table

The table below lists the longitude correction for common birth cities relative to their standard time zone (4 minutes per degree from the zone's reference meridian). Full true solar time also adds the equation of time for your birth date (within about ±16 minutes). Use the converter above, or enter your birth city in the BaZi calculator and it is applied automatically.

Note the pattern: coastal eastern China and Taiwan need almost no correction, while Singapore and Malaysia (UTC+8 despite lying near 100°E) subtract more than an hour, so births there very often shift into a different two-hour period. The values for New York, Los Angeles, London, Sydney, and Vancouver assume standard (winter) time: for a birth during daylight saving time, subtract one hour from the clock time first.

Worked Example: Same Clock Time, Different Hour Pillar

Take three births at exactly 9:10 AM clock time on 15 June 1998. In Taipei, true solar time is 9:16 AM — still within the Si hour (巳, 09:00–11:00), so the hour pillar is unchanged. In Hong Kong, the −23-minute longitude correction plus the equation of time gives 8:46 AM, crossing back into the Chen hour (辰, 07:00–09:00). In Singapore the correction is larger still: true solar time is 8:05 AM, also Chen. Identical clock times, yet the Hong Kong and Singapore charts take a different Hour Pillar — and in Ziwei Doushu a different palace layout — from the Taipei chart. This is why an accurate chart needs the birthplace, not just the birth time.

The Twelve Shichen and Their Clock Ranges

After converting to true solar time, the corrected time maps to one of twelve two-hour periods (shichen, 時辰). The Hour Pillar's Earthly Branch is the branch of that period.

Fortune Cloud's Approach

Fortune Cloud automatically applies True Solar Time correction using your birth city's longitude and the astronomical equation of time for your birth date. This programmatic approach eliminates manual calculation errors common in traditional practice. Your Major Cycles, Annual Fortune, and all derived readings start from an accurate foundation.