How it works

The date and clock time you enter are read at the IANA time zone you pick, using your browser's own time-zone data, so past daylight saving rules and old offsets apply. A clock time that was skipped when clocks went forward is rejected with a message, because no such moment existed. A clock time that happened twice when clocks went back uses the first occurrence, and the chart says which offset it chose.

The solar terms are real astronomical moments, so the birth moment is compared with them in UTC. The day and hour pillars then use the local date and clock.

This calculator changes the year pillar at the exact moment of Lichun, the solar term that opens spring. The older works checked for this site imply that boundary without stating it. The month pillar changes at the exact moment of each of the twelve 節, the major solar terms that begin the twelve months of the solar year. The table of those moments for 1900 to 2100 is generated in advance and sent to the browser only when the calculator is used.

Within a year, the month stem follows the rule called 五虎遁, which fixes the first month's stem from the stem of the year.

The day pillar comes from one unbroken count of days through the cycle of sixty. The hour pillar takes one branch for each two-hour block of the local clock and finds its stem from the rule called 五鼠遁, based on the stem of the day.

The block from 23:00 to 01:00 belongs to the 子 branch, and 23:00 to midnight is the late Zi hour. Modern schools disagree about it, and the older works checked for this site state no rule. By default this calculator changes the day pillar at midnight and takes the hour pillar's stem from the next day, the convention modern practitioners call 夜子時. If a birth falls in that hour, the chart also shows the alternate in which the day changes at 23:00, which changes the day pillar and leaves the hour pillar as it is.

The default is the clock time at the birthplace. Some schools prefer true solar time, the sun's actual position at that longitude. If you give a longitude, the calculator works out the longitude correction and the equation of time and tells you whether that reading would change the day or hour pillar. It leaves the year and month pillars alone, because those depend on the moment of the term and not on where you were standing.

A birth within 30 minutes of a 節 is flagged, because a recorded time is rarely that exact. The chart shows the pillars on the other side of the term. For a date with no time, a term falling on that date is flagged the same way.

Without a birth time there is no hour pillar, so the chart has three pillars. The year and month are read at local noon on that date, and a term that falls on the date is flagged. A birth at 23:00 or later on the given date would change the reading, and the calculator cannot know that from the date alone.

Every pillar shows its stem and branch with element and yin or yang, its 納音, its hidden stems 藏干, the ten god of its stem and of each hidden stem relative to the day master, and the day master's position in the twelve-stage cycle at that branch. Five-element counts use only the visible stems and branches. Relations among the branches use the same tables as the Chinese zodiac pairs. The 藏干 table follows one common modern arrangement, main stem first. The older sources list the same stems in other orders, and 三命通會 gives a different scheme by days of the month. Yin stems run backward through the branches, a rule 滴天髓 accepts and says not to cling to. The life stages place earth with fire, so 戊 has 長生 at 寅; some old sources place earth with water, and the chart says so for an earth day master.

大運 are the ten-year periods of the traditional formula. It uses the sex recorded at birth and the polarity of the year stem: yang-year men and yin-year women count forward from the month pillar to the next 節, and yin-year men and yang-year women count backward to the previous 節. The distance in days is turned into a starting age at three days to a year, shown in years and months. If you choose Skip, no luck pillars are made, and nothing else in the chart changes.

The calculator offers no verdict that a chart is strong or weak, and it names no favorable element, because schools disagree on those judgments. It describes a traditional system and leaves the conclusions to you.