How exact age is calculated
Your age in years is the number of birthdays you have had. The calculator subtracts the birth year from the year of the "age on" date, then takes one off if this year's birthday has not arrived yet. For the months and days it works the same way at a finer level: count complete months since the last birthday (if the day of the month has not been reached, that month is not complete), then count the days from the month anchor to the target date. The result is what a doctor's office, a school district or an immigration form means by age: for instance, 36 years, 8 months and 23 days.
Because actual month lengths are used, the answer stays consistent at the edges. Someone born January 31 is 1 month old on February 28 (or 29 in a leap year) and 1 month and 1 day old on March 1. Total days alive is a straight count of midnights between the two dates, so the birth day itself is not included; on your first birthday you have been alive 365 days (366 if the year contained February 29).
Next birthday, leap-day babies and day of the week
The next-birthday line checks whether this year's birthday is still ahead; if not, it moves to next year and tells you the weekday it falls on and the age you will turn. People born on February 29 get a birthday only every four years, so in the other years the tool uses February 28. Some states, and many people, use March 1 instead; legally, most US jurisdictions treat a leap-day birthday as reached on March 1 in common years for purposes like turning 21, but the difference is a single day.
The day of the week you were born is computed from the calendar directly, so you can settle the "were you a Tuesday's child" question or check what day a historical birthday fell on, back to 1900 and beyond.
Zodiac signs
The Western zodiac sign depends only on month and day: Aries March 21 to April 19, Taurus April 20 to May 20, Gemini May 21 to June 20, Cancer June 21 to July 22, Leo July 23 to August 22, Virgo August 23 to September 22, Libra September 23 to October 22, Scorpio October 23 to November 21, Sagittarius November 22 to December 21, Capricorn December 22 to January 19, Aquarius January 20 to February 18 and Pisces February 19 to March 20. Sources disagree by a day at some boundaries.
The Chinese zodiac assigns one of twelve animals to each lunar year. The tool uses the calendar year of birth (1900 was a Rat year, so the cycle repeats every twelve years), which is right for anyone born after Lunar New Year. Births in January or early February may belong to the previous year's animal; someone born January 20, 2025 is a Dragon, not a Snake, because the Snake year began January 29. Check the Lunar New Year date for your birth year if you were born in that window.