Time Zone and Daylight Saving Time: How Input Is Processed

The correct handling of clock time is a recurring source of error in four-pillar calculation. A recorded birth time is not automatically the time that should be used for the sexagenary hour pillar. Whether the record reflects standard time, daylight saving time, local mean time, or some other convention affects the conversion. This entry explains how this knowledge base treats time zones and daylight saving time, the difference between clock time and true solar time, and the fixed order of conversion steps.

钟面时间与四柱之间的换算

The charting engine used on this site accepts only Gregorian year, month, day, hour, minute, and sex as direct input. There is no longitude field. Times are interpreted as Beijing clock time, meaning the standard time of the East Eighth Zone based on 120 degrees east longitude. The engine does not internally convert to true solar time. Conversion to true solar time is treated as part of input preparation and must be completed before the data is entered into the engine.

The reason for this division is that the engine cannot know the birthplace of the person whose chart is being calculated. Since true solar time depends on longitude and date, it cannot be derived from a clock time alone. The user or the consulting practitioner must resolve the recorded time into the intended standard before input.

Longitude and time are related by the earth’s rotation. The earth rotates once in approximately twenty-four hours, and the full circle of longitude is 360 degrees. One hour therefore corresponds to 15 degrees of longitude, and one degree of longitude corresponds to about four minutes of time. This is the basis for comparing local solar time at different longitudes.

China officially uses one standard time across the whole country: the time of the 120th meridian east. All clock time in China is set to this standard regardless of local longitude. A city such as Urumqi lies at about 87 degrees east, which is 33 degrees west of the 120-degree standard meridian. At four minutes per degree, the difference is about 132 minutes. This means that when clocks in Urumqi show a certain Beijing time, the local solar time is about two hours and twelve minutes behind the clock reading.

The equation of time is a separate adjustment. Over the course of a year it ranges from about minus fourteen minutes to plus sixteen minutes. It depends only on the date, not on the birthplace. On a given day the value of the equation of time is the same everywhere in China. It arises from the earth’s elliptical orbit and axial tilt, not from the observer’s location.

The total difference between clock time and true solar time therefore has two components. The longitude correction depends on birthplace; once the place is fixed, that component is constant throughout the year. The equation of time depends on the date and is independent of location. The two must be handled separately and then added. Treating them as a single combined correction can hide errors.

夏令时：哪些年份与地区实行过

Daylight saving time was used in mainland China from 1986 through 1991. In each of those years it began around mid-April and ended around mid-September, though the exact start and end dates varied slightly from year to year. During the daylight saving period, clocks were set forward by one hour from Beijing standard time.

For a person born in mainland China during those six years, the recorded clock time may already include the daylight saving shift. If the recorded time is a clock time and the birth occurred within the daylight saving period of that year, one hour should be subtracted first to restore Beijing standard time. After that subtraction the time can be treated as standard Beijing clock time.

The subtraction of one hour for daylight saving time is not the same as a longitude correction. The two steps are independent. One restores the clock reading to the standard time of the time zone; the other adjusts for the difference between the standard meridian and the birthplace meridian. They cannot replace each other, and they should not be merged into a single rough addition or subtraction. If they are merged, the result may accidentally double-count or cancel out part of the correction.

The exact dates for each year should be checked against a reliable calendar of Chinese daylight saving time. Because the period was not identical across all six years, assuming a single fixed start and end date can introduce a one-hour error for births near the boundaries.

出生地与居住地时区不同

When a person is born in one time zone but the record was kept according to another time zone, the recorded time must first be converted to the time zone intended for charting. This knowledge base uses Beijing time as the standard for all mainland Chinese births. If a birth record uses a different zone—for example, a local time used in a border region, or a time kept by a family according to a foreign standard—the time must be converted to Beijing time before input.

The same applies to births outside mainland China. If the engine is used for a birthplace outside China, the practitioner must first decide which standard time will serve as the input basis. The engine itself does not convert between zones. The user must normalize the time before entering it.

A common error is to take a birth time recorded in local civil time in another country and enter it directly as if it were Beijing time. This silently shifts the hour pillar. The correct procedure is to convert the recorded local time to the chosen input standard first, then apply the birthplace longitude correction for true solar time if that is part of the practitioner’s method.

跨时区与在途中出生

A birth that occurs while traveling, at sea, or in the air raises a separate problem of which location should define the birth time. The record may show the time according to the departure zone, the destination zone, or the zone of the vehicle’s present position. Since the engine has no longitude field and no event location field, this ambiguity cannot be resolved inside the engine.

For such cases the practitioner should record the time as given, note the location as precisely as possible, and determine which time standard the record used. Only after the event is fixed to a place and a time standard can the longitude correction and equation of time be meaningfully applied.

If the exact location is not known, or if the record does not state whether the time is local or Beijing time, the data should be marked as pending verification. A chart calculated from an unresolved in-transit time is provisional.

真太阳时属于输入准备

True solar time conversion is not a function of the charting engine. It belongs to the stage before data entry. This is because true solar time requires longitude and date, and the engine deliberately accepts no longitude. The user must prepare the time so that the input fields contain the intended standard time.

The fixed order from a recorded clock time to the hour pillar is four steps. First, remove daylight saving time if the birth record falls within a daylight saving period. Second, unify the time zone, converting the time to Beijing standard time if necessary. Third, convert to true solar time using the birthplace longitude and the equation of time for the date. Fourth, look up the result in the twelve double-hour table to obtain the hour pillar.

Reversing this order causes identifiable errors. The most common consequence is that the one-hour daylight saving removal and the longitude time difference are double-counted or cancel each other. For example, if a practitioner subtracts one hour for daylight saving time and then also applies a longitude correction that was already partially embedded in the record, the final time may drift by an hour or more. If the steps are merged into a single rough offset, the seasonal variation of the equation of time is also lost.

Before charting, four input items should be gathered: the time as written on the record; the birthplace to city or district level; whether the record uses local time or Beijing time; and whether the birth year and place had daylight saving time or a time zone change. If any one of these is missing, the time should be marked as pending verification. A chart produced from incomplete time data remains provisional until the missing item is confirmed.