True Solar Time Calculator

True Solar Time Calculator

LEARN BAZI TOOL

True Solar Time Calculator

Convert between recorded civil time and True Solar Time, or find the local clock window for a selected Chinese hour.

Advanced time-zone and location settings

Use a manual offset only when you have a reliable historical source or the recorded clock followed a special convention.

Uses True/Apparent Solar Time. Historical offsets are derived from the server’s IANA time-zone database and should be independently verified for disputed, wartime, or pre-standard-time records.

How to Use the True Solar Time Calculator

This calculator converts between local civil clock time and True Solar Time (TST). It supports two calculation modes:

  • Historical Solar Time — converts a recorded local date and time into True Solar Time and identifies the corresponding 时辰 Shí Chén — Chinese two-hour period.
  • Date Selection Solar Time — converts a selected Shí Chén into the local civil-clock window for a chosen date and location.

1. Historical Solar Time

Use this mode when you already have a recorded date and clock time, such as a birth time or the time of a past event.

  1. Select Historical Solar Time.
  2. Enter the recorded local date and time.
  3. Choose the estimated accuracy of the recorded time.
  4. Search for the city or location where the event occurred.
  5. Select the correct location from the search results.
  6. Check that the displayed country and time zone are correct.
  7. Click Calculate Solar Time.

The result includes:

  • The recorded local civil time
  • The applicable UTC offset
  • Local Mean Solar Time
  • The Equation of Time adjustment
  • The calculated True Solar Time
  • The corresponding Shí Chén
  • A boundary-sensitivity assessment
  • Any daylight-saving or historical time-zone warnings

Select View detailed calculation if you want to see the technical calculation information.

Recorded-Time Accuracy

Version 0.7 allows you to indicate how accurately the original time was recorded.

Choose the option that most closely describes your information:

  • Exact / within 1 minute — use when the time came from a reliable digital record, official record or precisely noted clock time.
  • Rounded to the nearest 5 minutes — use when the recorded time may have been rounded slightly.
  • Rounded to the nearest 15 minutes — use when the time is approximate to the nearest quarter-hour.
  • Approximate — use when the recorded time may differ by up to approximately 30 minutes.
  • Unknown — use when the reliability of the recorded time is uncertain.

The calculator tests the possible time range associated with your selection. It will indicate whether the Shí Chén classification remains stable or could change near a boundary.

Understanding Boundary Sensitivity

A Shí Chén changes every two solar hours. A small uncertainty in the recorded time can therefore affect the result when the calculated True Solar Time is close to a boundary.

The calculator uses the following ratings:

  • Safe — more than 15 minutes from the nearest Shí Chén boundary
  • Caution — between 5 and 15 minutes from the nearest boundary
  • Critical — less than 5 minutes from the nearest boundary
  • On boundary — the calculated time falls directly on or extremely close to a boundary

A result may also be described as:

  • Stable — the entire tested time range remains within the same Shí Chén.
  • Boundary-sensitive — the tested range crosses a boundary, so two different Shí Chén classifications may be possible.

For boundary-sensitive results, check the reliability of the original recorded time before using the result for detailed BaZi analysis.

Daylight-Saving Time and Clock Changes

The calculator checks the selected location’s historical time-zone information.

If the entered clock time occurred during a daylight-saving transition:

  • A non-existent time may be detected when clocks moved forward and skipped a period of local time. You will need to correct the entered time.
  • A duplicated time may be detected when clocks moved backward and the same clock time occurred twice. You will be asked to choose the first or second occurrence.

When available, the applicable historical UTC offset and clock-transition information will be shown in the result.

2. Date Selection Solar Time

Use this mode when you have chosen a date and Shí Chén and want to know the corresponding local clock time at a particular location.

  1. Select Date Selection Solar Time.
  2. Choose the desired date.
  3. Select the required Shí Chén.
  4. Choose a safety buffer.
  5. Search for the intended location.
  6. Select the correct location from the search results.
  7. Click Calculate Solar Time.

The result card displays:

  • The preferred local civil date and time
  • The selected Shí Chén
  • The selected location
  • The applied safety buffer

Select View detailed calculation to see the complete unbuffered local clock interval, midpoint, solar-time adjustments and time-zone information.

Safety Buffer

The exact beginning and end of a Shí Chén are boundary points. Small differences in coordinates, historical time-zone records, clock accuracy or calculation methods may affect times close to these boundaries.

The safety buffer shortens the recommended window by removing time from both ends of the exact interval.

For example, a 15-minute buffer means that the preferred window begins 15 minutes after the calculated start and ends 15 minutes before the calculated finish.

A larger buffer provides more separation from the boundaries but creates a shorter usable window.

Important Note for 子 Zǐ — Rat Hour

Zǐ hour spans 23:00 to 01:00 in True Solar Time and therefore crosses midnight.

For date selection, the chosen date represents the solar date on which Zǐ hour begins. Depending on the location, longitude, Equation of Time and UTC offset, the converted local civil-clock window may:

  • remain on the selected civil date;
  • cross from the selected civil date into the following date; or
  • fall mainly on the following civil date.

When the preferred local window crosses civil midnight, the result displays both dates clearly, for example:

20 September 2026 → 21 September 2026
11:45 PM – 1:15 AM

Always follow the dates and times shown in the result card rather than assuming that the entire Zǐ-hour window belongs to one civil calendar date.

Choosing the Correct Location

Select the location where the birth, event or planned activity physically occurred. Do not use your present location unless it is also the location of the event.

If several places have similar names:

  • check the country or region;
  • check the displayed time zone;
  • choose the nearest appropriate city; and
  • use exact coordinates when greater location precision is required.

Longitude directly affects solar-time conversion, so selecting the correct location is important.

Advanced Time-Zone and Location Settings

Open Advanced time-zone and location settings when you need greater control over the calculation.

Automatic UTC offset

This is the recommended setting for most users. The calculator uses the selected location, date and available historical time-zone records to determine the applicable UTC offset, including daylight-saving changes where supported.

Manual UTC offset

Use this only when you have a reliable historical source confirming that the official local offset differs from the automatic result.

A manual offset overrides the automatic time-zone calculation. Incorrect use may significantly change the result.

Coordinate source

You can choose between:

  • Matched city coordinates — uses the coordinates supplied by the selected location result.
  • Exact coordinates — allows you to enter the precise latitude and longitude of the event location.

Use exact coordinates for rural places, large metropolitan areas or locations far from the selected city centre.

For longitude:

  • East is positive.
  • West is negative.

For latitude:

  • North is positive.
  • South is negative.

The exact coordinates should remain within the same time-zone region as the selected location. Coordinates do not automatically change the legal time zone.

Historical Accuracy

For Singapore and Malaysia, the calculator includes curated historical UTC-offset information for supported periods. Other locations generally rely on the available IANA Time Zone Database.

Historical time-zone accuracy can vary because:

  • governments changed legal time standards;
  • daylight-saving rules changed over time;
  • older records may be incomplete;
  • borders and time-zone regions changed;
  • some recorded times were approximate or rounded; and
  • local clocks were not always accurately synchronised.

The calculator displays its time-zone data status where available. For historically sensitive cases, compare the result with reliable official or archival sources.

Interpreting the Result

The calculated Shí Chén is based on True Solar Time, not directly on the clock time originally entered.

A clock time and its True Solar Time may differ because of:

  • the location’s longitude within its official time zone;
  • the Equation of Time;
  • the historical UTC offset;
  • daylight-saving time; and
  • historical legal clock changes.

This means that the calculated True Solar Time may fall on a different hour—or occasionally a different civil date—from the original recorded clock time.

BaZi Convention Note

This calculator provides solar-time conversion and Shí Chén classification. It does not determine the Day Pillar or decide between different BaZi lineage rules concerning early and late Zǐ hour.

If your practice uses a special convention for changing the BaZi day at 23:00, 00:00 or another point, apply that convention separately after reviewing the calculated True Solar Time.

Accuracy and Use Disclaimer

This calculator is provided for educational and informational purposes only. Results depend on the accuracy of information entered, geographic coordinates, historical time-zone records, daylight-saving rules, astronomical formulas and third-party data. Historical records may be incomplete, disputed or subject to different interpretations, and calculation or system errors remain possible. Users should independently verify important results with reliable sources or a qualified practitioner. By using this calculator, you accept responsibility for its application and agree that Learn BaZi and its operators shall not be liable for any loss, decision or consequence arising from its use.