4 minutes per degree
Longitude correction = 4 × longitude − 60 × UTC offset. This compares the birthplace with the time-zone reference meridian.
FREE LOCAL TOOL · EQUATION OF TIME + LONGITUDE
True solar time calculator.
Check your birth-time window.
Compare recorded local time with an approximate true solar correction. See the corrected date, time and two-hour window, with each calculation step shown before you use it in a chart.
CALCULATE
Use the offset
from that date.
Enter east longitude as positive and west longitude as negative. The UTC offset must be the historical offset used by the birthplace, including DST where applicable.
THE FORMULA
Two corrections,
shown separately.
This educational approximation follows the commonly published NOAA fractional-year equation. It is useful for checking whether a birth time sits near a traditional boundary, not for legal timekeeping or astronomical certification.
Seasonal solar variation
A fractional-year approximation accounts for the apparent Sun running ahead of or behind mean clock time during the year.
Add, then inspect the boundary
Total correction = longitude correction + equation of time. Ziheng shows a date shift and traditional window change explicitly.
γ = (2π / days in year) × (day of year − 1 + (hour − 12) / 24)
EoT = 229.18 × (0.000075 + 0.001868 cos γ − 0.032077 sin γ − 0.014615 cos 2γ − 0.040849 sin 2γ)
true solar time ≈ civil time + EoT + 4 × longitude − 60 × UTC offsetReference: NOAA Solar Calculation Details. NOAA describes these equations as approximations; this tool rounds the total correction to the nearest second.
USE IT CAREFULLY
Correction is a choice,
not a universal rule.
Chinese astrology schools differ on whether and how to apply true solar time. Keep the civil record, historical offset, longitude, corrected result, and chosen convention together.
Compare both charts
If correction crosses 01:00, 03:00, or another two-hour boundary, calculate both versions and identify what actually changes.
Current offset may be wrong
A city’s present UTC offset does not prove the offset used on a historical birth date. Verify local time-zone and daylight-saving records.
Do not manufacture precision
If the recorded hour is uncertain, solar correction cannot repair the source record. Compare plausible windows and label uncertainty.
FAQ
Before you
change the chart.
What UTC offset should I enter?
Use the offset that applied at the birthplace on the recorded date, including daylight-saving time. Do not automatically use your current device’s offset.
Is true solar time mandatory for BaZi or Zi Wei Dou Shu?
No. Traditions and practitioners differ. Ziheng treats it as an optional, visible comparison and preserves the original civil time.
Does the calculator upload my inputs?
No. The calculation runs locally in your browser. The form values are not sent to Ziheng, Stripe, or analytics.
Cite the method: Ziheng, “True Solar Time Calculator for BaZi & Zi Wei Dou Shu,” updated 25 September 2026.Read full methodology →