How this calculator works
Choose age mode (birth date to today) or duration mode (any two dates), enter the date(s), and this calculator computes the span both as years/months/days and as total days and weeks.
The formula
years = endYear − startYear months = endMonth − startMonth days = endDay − startDay If days < 0: months -= 1; days += (days in the month immediately before the END date's month) If months < 0: years -= 1; months += 12 totalDays = exact calendar day count between the two dates totalWeeks = floor(totalDays / 7), remainder totalDays % 7
Convention note: when the day subtraction goes negative, this calculator borrows days from the month before the later date (not the earlier one). For example, January 31 to February 28 computes as 0 months and 28 days, not 1 month and 0 days, which is an equally valid convention other calculators sometimes use.
Worked example
From January 31, 2000 to February 28, 2026:
- Years: 26, Months: 2 − 1 = 1, Days: 28 − 31 = −3 (negative, so borrow)
- Borrow from January (31 days): months → 0, days → −3 + 31 = 28
- Result: 26 years, 0 months, 28 days
Frequently asked questions (FAQ)
Why do some age calculators give a different answer for the same dates?
Calculating years/months/days between two dates has a genuine ambiguity when month lengths don't line up evenly, for example, from January 31 to February 28. Different calculators resolve this differently; this one's convention is documented below.
Does this account for leap years?
Yes. All date math (including someone born on February 29) correctly accounts for leap years, since it's based on real calendar dates rather than a fixed 365-day approximation.
What's the difference between "age" mode and "duration" mode?
Age mode always measures from a birth date to today. Duration mode lets you pick both a start and end date, useful for measuring the length of an event, project, or any span of time that isn't tied to "now."
How many total days old am I?
This calculator shows your exact total day count alongside the years/months/days breakdown, useful for "X,000-day" birthday milestones some people like to celebrate.
Does this calculator use my timezone?
No. All dates are treated as plain calendar dates (year-month-day) rather than moments in time, so the result doesn't depend on your timezone or daylight saving time. Entering a date always means that calendar day, everywhere.
Sources
This is standard calendar date arithmetic; no external source is needed. The month-length-asymmetry convention is a deliberate, documented choice (see above).