How to Use Birthday Calculator
The Birthday Calculator turns a single date into a colorful summary.
- Enter birth date.
- Pick a target date — Today by default.
- Read age, weekday of birth, weekday of next birthday, days lived and zodiac signs.
Formula & Theory — Birthday Calculator
ymd = diffYMD(birth, target)
totalDays = floor((target − birth) / 86400000)
totalWeeks = floor(totalDays / 7)
totalMonths = years × 12 + months
birthDow = weekday(birth)
nextBirthday = next occurrence of (birth.month, birth.day) ≥ target
nextDow = weekday(nextBirthday)
zodiac = ZODIAC_RANGES.find(r => r.contains(birth.month, birth.day))
chineseZodiac = CHINESE[(birth.year) mod 12]| Symbol | Meaning |
|---|---|
| ymd | Y/M/D age |
| birthDow | Day-of-week the person was born |
| nextDow | Weekday of the next birthday |
| zodiac | Western zodiac sign |
| chineseZodiac | Chinese zodiac animal |
The Birthday Calculator combines math and folklore so the result feels personal.
Use Cases for Birthday Calculator
- Birthday cards — Add fun zodiac trivia.
- Party planning — Know what day of the week the next birthday lands.
- Social posts — Share interesting “days lived” milestones.
- Children memory books — Capture birth weekday and zodiac.
- Astrology fans — Quick lookup for zodiac signs.
- Cultural calendars — Discover Chinese zodiac year.
The Birthday Calculator is a tiny tool with a lot of personality.