How Birthdex works out every answer

The zodiac sign

We use the Western tropical zodiac with the common calendar ranges, for example Cancer from June 21 to July 22. These ranges are a rounding. The real boundary is the moment the sun enters the next sign, which moves by up to a day between years, so a birthday on or beside a boundary can belong to either sign depending on the year and the time zone. We do not calculate that moment. Instead, boundary dates are flagged on their own pages and every boundary is listed in the cusp guide. The exact ranges we hold are published at /api/zodiac.json, so you can check them rather than take our word for it.

The birthstone

The primary stone per month follows the list in common jewelry-trade use today. Each month also carries its recognized alternatives and, where it differs, the older traditional stone. Every stone has its own color field, so a month with several stones can never be described using another stone's properties.

Unverified: the modern list is widely credited to a standardization by the American jewelry trade in 1912, with later additions. We have not read that original document, so we do not cite it as a source and you should not quote this page for that date. What we can stand behind is that the values in /api/birthstones.json reflect current common usage.

The birth flower

Each month has a primary and a secondary flower from the Western birth-flower tradition, with meanings from Victorian floriography. Several competing nineteenth-century flower dictionaries were in circulation and they disagree with each other, so these meanings are the common reading rather than a definitive one. We have not reproduced any single dictionary.

The weekday and the date arithmetic

Weekdays are computed in the proleptic Gregorian calendar using UTC, so the answer never depends on the timezone of the device rendering the page. Leap years follow the full rule including the century exception, which means 1900 was not a leap year and 2000 was. Day-of-year positions are counted over a 366-day year so that 29 February has a place in the sequence. All of this is covered by unit tests in the repository, including the edge cases.

The compatibility percentage

It is a transparent arithmetic model based on element and modality, not a reading and not a prediction about any real relationship. The full formula, with the points awarded at each step, is set out on the compatibility method page, and every pair page shows the calculation that produced its own number so you can check it.

What this site will not claim

Astrology, birthstone lore and flower meanings are cultural traditions with long and interesting histories. They are offered here for interest. Nothing on this site claims that a birth date determines anyone's character, health, life expectancy, compatibility or future, and nothing here should be read as advice. Where the site presents a statistical claim it names the fact that we have not verified the underlying data, rather than dressing it up with a source we did not read.

Corrections

If something here is wrong, tell us and we will fix it and note the date. The address is on the contact page. Corrections are the reason every page carries a review date rather than a build date.

Frequently asked questions