How to Fill in Birth Time: Daylight Saving Time, True Solar Time, and the Assignment of the Hour of the Rat (子時) to a Day

The Time on Your Birth Certificate Is What You Should Enter

The single most important rule for filling in your birth time is: Enter the time exactly as it appears on your birth certificate. Do not convert it yourself.

Based on our site's statistics, this is the most common source of user error. Some people hear they need to convert to "true solar time," so they look up the longitude, calculate the equation of time, and end up making a mistake. Others are unaware that DST was in effect in their birth year and manually adjust the time, resulting in the Hour Pillar (時柱) being off by a full two-hour period (時辰).

The birth certificate you hold contains the time that was recorded by the hospital or household registration office at the time. This recorded time is the input value the charting system needs.

What the System Handles for You

You only need to enter the time from your birth certificate and your birth city. The system will automatically perform the following conversions behind the scenes:

1. Daylight Saving Time Identification and Reversal

Taiwan implemented DST (then called "Daylight Saving Time" (日光節約時間)) multiple times between 1945 and 1961. Mainland China implemented DST from 1986 to 1991. During the summers of these years, the clocks on birth certificates were set one hour ahead.

The system automatically determines whether DST was in effect at your location based on the birth date and city you enter. If it determines DST was active, the charting page will note, "DST was in effect in this location at the time of birth," and will reverse the time by one hour before proceeding with the subsequent calculations.

If you manually subtract one hour before entering it, and the system subtracts it again, the Hour Pillar will be off by a full two-hour period.

2. True Solar Time Conversion

The "true solar time" (真太陽時) used in bazi charting is intended to reflect the actual position of the sun at the place of birth. Its calculation formula is:

True Solar Time = Standard Time + Longitude Time Difference + Equation of Time

The two variables are:

• Longitude Time Difference (經度時差): For each degree of longitude difference, there is an approximate four-minute time difference. There is a discrepancy between the standard meridian used by Taiwan and the local longitude of various places.
• Equation of Time (均時差): Because Earth's orbit is elliptical and its axis is tilted, the length of a true solar day is not constant throughout the year. The difference between this and the average solar day is called the "equation of time," fluctuating between plus or minus sixteen minutes.

Some websites or tools only perform the longitude correction, ignoring the equation of time. In most cases, this has little impact. However, if your birth time happens to fall near the boundary of a two-hour period, a sixteen-minute deviation from omitting the equation of time is enough to change the Hour Pillar.

The system looks up the longitude based on your birth city and then incorporates the equation of time corresponding to your birth date, completing the correction in one step. You do not need to do it yourself.

3. Assignment of the Hour of the Rat (子時)

The Hour of the Rat (子時) spans midnight, so a day is actually divided into thirteen segments: the Early Hour of the Rat (早子時, 00:00–00:59) and the Late Hour of the Rat (晚子時, 23:00–23:59), which have different ganzhi (干支) designations.

Traditionally, for someone born in the Late Hour of the Rat, there are two practices regarding whether the Day Pillar (日柱) should be calculated as that day or the next day:

• Late Hour of the Rat assigned to the current day: The Day Pillar remains the current day's pillar. This is the common practice.
• Late Hour of the Rat assigned to the next day: The Day Pillar changes to the next day's from 23:00 onward.

Both practices have their own reasoning, and there is no definitive conclusion. Our site has made this an option, allowing users to decide which school of thought to follow.

How to Fill in the Place of Birth

The Place of Birth field should be filled in with the city at the time of birth, not your current place of residence.

The purpose of this field is to look up the longitude and determine if it was in a DST region. If you were born in Kaohsiung and later moved to Taipei, please enter Kaohsiung. If you were born in a city in mainland China, please enter the name of that city.

Common Sources of Error

Below is a summary of the most common reasons for incorrect time entry:

Remember These Two Things

When filling in your birth time, please remember two things:

1. Enter the information exactly as it appears on the birth certificate, without any manual conversion.
2. Enter the city of birth for the place of birth, not your current residence.

Daylight Saving Time reversal, true solar time calculation, and equation of time correction—these are all handled automatically by the system after you press the charting button. Your job is simply to provide the correct raw data.