Calculation methodology
Weekday calculations
The weekday is determined from your browser’s local calendar interpretation of the entered date. We avoid parsing YYYY-MM-DD as UTC, which can shift the calendar day.
Leap years and February 29
Leap years follow the Gregorian rule. If you were born on February 29, non-leap-year birthdays are observed on February 28 for previous/next birthday and age-anchor calculations.
Age calculations
Age in years, months, and days uses the browser’s current local date. Hours, minutes, and seconds alive are approximate: birth is treated as local midnight on the entered date because version 1 accepts a date only. The seconds total can update live in the results view.
Moon phase
Moon phase at birth uses a mean synodic-month model evaluated at local noon on the birth date. It is an educational approximation, not an ephemeris-grade astronomical calculation.
World population age share
The estimate of how much of the world’s population is older than you uses a simplified static snapshot of mid-2020s world age structure, rounded from UN World Population Prospects 20245-year age groups. It is not a live census lookup and will drift as the true age structure changes.
The histogram is typically young-heavy (a classic global age pyramid). That mostly reflects higher birth cohorts in many regions plus mortality across the lifespan. It isnot primarily a “Baby Boomers dying out” shape. That bulge is more visible in countries like the United States. Globally, the median age is still around 30.
Western zodiac
Western zodiac signs use common tropical date ranges. Capricorn wraps the year boundary. Exact historical cusp definitions vary.
Chinese zodiac
Chinese zodiac animals follow the traditional twelve-year cycle. The animal changes at Chinese New Year (lunar month 1, day 1), which usually falls between late January and mid-February on the Gregorian calendar. Births before that year’s New Year keep the previous animal. New Year dates for 1800 to 2100 are baked into the calculator from standard lunar conversion tables.
Birthstones and generations
Birthstones follow a modern Western jewelry tradition. Generation labels follow common U.S. popular and research windows (including Pew’s Millennial years of 1981 to 1996). Other writers may shift those edges by a few years; see the generations pages for the map this calculator uses.
What is factual vs entertainment
- Factual: weekday, age, day counts, leap-year experience, calendar milestones
- Approximate / educational: hours/minutes/seconds alive (no birth time), moon phase, world population age share
- Cultural / entertainment: Western zodiac, Chinese zodiac, birthstones, generational labels