Facts, dates, and traditions need different labels

Flowers and stones

The calculator maps the selected month to the primary flower and stone used in our maintained dataset. Entity and month pages also identify recognized alternatives and explain when lists vary by organization, country, or period.

Modern and traditional birthstone lists

Two lists are in common use and we record both rather than choosing silently. The modern American list was standardised by the National Association of Jewelers in 1912, which favoured transparent, faceted gems so that several children's stones could be set in one ring. It was revised in 1952 (alexandrite for June, citrine for November, tourmaline for October, zircon for December), in 2002 (tanzanite for December) and in 2016 (spinel for August). The older traditional list preceding it gives bloodstone for March, sardonyx for August, opal for October, topaz for November, and turquoise or lapis lazuli for December.

Six months (January, February, April, May, July, September) name the same single stone on both lists. Three (March, August, December) lead with a different stone on each. Three (June, October, November) kept the traditional stone as the lead while the modern list added options. Because the 1912 change was commercial rather than gemological, neither list is more authoritative, and a birthstone assignment is a published convention rather than a property of the gem or the month.

Western zodiac and boundary drift

The calculator uses a maintained set of conventional tropical-zodiac date ranges. It does not calculate an astronomical birth chart. A tropical sign begins the moment the Sun reaches its longitude on the ecliptic — 0° Aries is the March equinox — and because the tropical year is about 365.2422 days long that moment moves by up to roughly a day between consecutive years, resetting with the leap-year cycle.

A published start date is therefore correct for some years and a day out for others, which is why reputable tables disagree. Sign pages show both days each boundary can fall on and state plainly that a birthday landing on one of them cannot be resolved from a date table; that needs the birth year, and strictly the birth time and time zone. We do not present a thirteenth "cusp" sign, because in astrological tradition the Sun occupies exactly one sign at any moment.

Chinese zodiac

The animal year changes at Chinese New Year, not January 1. For January and February dates, the calculator checks the maintained Chinese New Year boundary for the selected year before assigning the cycle year. It supports only years present in that maintained table and does not silently extrapolate beyond it.

Each animal page publishes the exact Gregorian span of every one of its zodiac years, derived from the same boundary table: the first day is that year's Chinese New Year and the last day is the day before the next one. An end date is shown only when the following year's boundary is also in the table, so no span is ever estimated. The animal repeats every 12 years and the five elements every 10, so a full animal-and-element pairing returns once every 60 years.