Date Arithmetic,
Exact to the Single Day.
Free age calculators: exact age in years, months, days — plus days lived, next-birthday countdowns, and date arithmetic.
Leap Year Arithmetic
Accounts for leap years and month day variations without rounding errors.
Days Lived Milestone
Calculate 10,000 days or 1,000 weeks milestones.
Day of the Week
Find the exact day of the week you were born.
AgeMath Tools
Exact Age Calculator
Your precise age in years, months, and days — plus total days lived and the next-birthday countdown.
Next Birthday Countdown
Days, hours, and weekends until the next birthday — plus which weekday it lands on and the half-birthday.
Date Difference Calculator
Days, weeks, months, and years between any two dates — with working-days and the countdown-or-since framing.
Days Lived & Life-in-Numbers
Your life in counters — days, hours, meals, heartbeats, and full moons witnessed.
Practical Guides & Best Practices
The birthday paradox, and why shared birthdays are common
Twenty-three people, fifty percent — the counterintuitive math of shared dates.
Day counting: the psychology of concrete numbers
Why 11,700 days motivates and 32 years does not — and the practices built on the difference.
12 mistakes people make with age math
The recurring errors we see in age math — what causes each one, and the habit that prevents it.