Enter your birthday to get your exact age and how many days until your next birthday, or find out how many days are between two dates.
| Days until next birthday | |
| Total days lived |
When people calculate date differences or age by hand, two details are easy to miss: leap years, and the fact that months don't all have the same number of days (28, 29, 30, or 31, depending). Whether "February this year" has 28 or 29 days depends on whether it's a leap year (a year divisible by 4 is a leap year, unless it's also divisible by 100 — in which case it isn't, unless it's also divisible by 400; so 1900 wasn't a leap year, but 2000 was). If you simplify by assuming every month has 30 days, the error compounds the further out you calculate. This tool sidesteps all of that by never manually subtracting years, months, and days at all — it converts both dates into JavaScript Date objects, subtracts them to get a millisecond difference, then divides by the number of milliseconds in a day (86,400,000) to get a day count. Because this relies directly on the browser's built-in calendar logic, leap years and variable month lengths are already handled correctly without any custom rules needed.
The most common mistake in age calculation is simply subtracting birth year from the current year and stopping there — ignoring whether this year's birthday has actually occurred yet. For example, someone born August 20, 1995 hasn't had their 2026 birthday yet as of March 2026 — naively computing 2026 − 1995 = 31 would be off by one. The correct approach checks whether today's month and day have reached or passed the birth month and day; if not, subtract 1 from the raw year difference. That's exactly what this tool's logic does: it computes the year difference first, then compares month and day, subtracting 1 if this year's birthday hasn't happened yet — which is why the result is a precisely calculated exact age, not a rough estimate.
Say the birthday is February 29, 2000 (a date that only exists in leap years), and today is August 8, 2026. First, the year difference: 2026 − 2000 = 26. Then check whether today (August 8) has passed the birth month/day (February 29) — it has, so the age stays at 26 with no adjustment needed. Days until the next birthday is calculated as the gap between today and "February 29 of this year (or next year, if this year's birthday already passed)." One subtlety: if the current year isn't a leap year, February simply doesn't have a 29th — JavaScript's Date object handles this by automatically rolling the date forward to March 1. That's native behavior of the Date object itself, and it's why someone born on February 29 will see their "birthday" land on March 1 in any non-leap year.
The "Date Difference Calculator" section subtracts the start date from the end date. If you pick an end date earlier than the start date, the raw math produces a negative number — but the tool displays the absolute value (always a positive day count) and adds a note underneath ("end date is before start date") so you know the order was reversed, rather than assuming the tool made an error. The day count is converted to weeks by rounding down (10 days shows as "1 week," not rounded up to 2), and converted to months using an average of 30.44 days per month (365.25 days ÷ 12 months) — so the month figure is always an approximation for quick reference, not an exact calendar-month count.
It's your exact age, calculated precisely from your actual date of birth — you don't gain a year until the day itself arrives. Traditional age-counting systems used in some cultures (where you're "1" at birth and gain a year at each new year) work differently and can differ from this result by a year or two — that's expected and not an error.
Under the hood this uses JavaScript's built-in Date object. If the specified year doesn't have a February 29, the date automatically rolls forward to March 1. So in a non-leap year, the "days until next birthday" calculation is based on March 1 — that's how the browser's date engine itself behaves, not a bug in the tool.
No — the day count always displays as a positive number (e.g. a 5-day gap shows as "5 days"), but the smaller text underneath adds a note that "the end date is before the start date," letting you know the order is reversed in case you picked the wrong field.
"Total days lived" is an exact count, computed directly from the millisecond difference with no rounding error. But "about X months" in the date-difference section is an estimate based on an average of 30.44 days per month, since actual months vary from 28 to 31 days — treat it as a rough reference, not an exact calendar-month count.
For the age calculator, nothing displays until you pick a date — the result area just stays blank, no error message. The date-difference calculator, on the other hand, automatically defaults both the start and end date to today when the page loads, so you'll see a "0 days" result immediately without entering anything.