Age is the measurement of time that has elapsed from a starting date of birth to a specific target date. While calculating age in simple years is straightforward, resolving the exact age in months, weeks, and days is surprisingly complex due to the irregularities of the calendar system. Month lengths fluctuate between 28, 29, 30, and 31 days, and leap years add an extra day to February every four years, making manual calendar subtraction tricky to solve.
To calculate age exactly, the math tracks the number of full years first, then full months, and finally the remaining days.
If the target day of the month is smaller than the birth day, the calculation borrows days from the preceding month, adjusting for the specific number of days in that month. To compare date differences across months or calculate specific target dates, use our dedicated date differences tool.
Leap years occur because the earth takes approximately 365.2422 days to orbit the sun. To keep our calendar aligned with the seasons, we add a leap day (February 29) every four years.
For people born on February 29 (leap day babies), calculating age on non-leap years requires a rule: legally, their age increases on March 1 in most jurisdictions. Our online age calculator handles leap years and variable month lengths automatically, ensuring exact results.
Suppose a child is born on February 29, 2020, and you want to calculate their exact age on March 15, 2026.
First, the calculator counts the full years: from Feb 29, 2020, to Feb 29, 2024, is 4 years. Since 2025 and 2026 are not leap years, we count to the nearest common day, Feb 28, 2026, which is 6 years. From Feb 28, 2026, to March 15, 2026: March has 31 days. The calculator adds 15 days of March plus the 0 days remaining in February, yielding exactly 6 years, 0 months, and 15 days. It also shows that the child has experienced only one actual leap day birthday. This shows how birth dates are adjusted to track true calendar intervals.
Calendar age should not be confused with elapsed time expressed as a decimal year. Months have unequal lengths, so six calendar months are not always exactly half of 365 days. Legal eligibility may begin at the start of a birthday, at a specific local time, or under a special rule set by the relevant jurisdiction. Medical records for infants often use completed weeks or days because those units are more precise for development. Choose the representation that matches the decision being made and retain the original birth and comparison dates.