Understand this tool
How completed age is calculated
- What the concept means
- Completed age is the number of anniversaries reached between a birth date and a comparison date.
- Why it exists
- It expresses age in whole years while also showing the calendar remainder after the last anniversary.
- When to use it
- Use it for ordinary civil-age calculations where the applicable jurisdiction follows the same anniversary convention.
- What the result means—and does not mean
- The result is a calendar interpretation, not an exact biological age or a universal legal ruling.
Age is not just a day count divided by 365
Calendar years have different lengths, so completed age changes on an anniversary rather than after a fixed 365-day duration. Months and days in a remainder are also calendar units, not constant lengths.
Leap-day birthdays require a convention in non-leap years. Laws and institutions may recognize February 28 or March 1 for particular purposes; this page’s arithmetic cannot decide that rule.
Key concepts
Key concepts
- Completed age
- Whole anniversaries reached by the comparison date.
- Anniversary
- The yearly calendar recurrence of a birth date.
- Exact duration
- Elapsed time measured in fixed units between instants.
- Leap-day birthday
- A February 29 birth date needing a non-leap-year convention.
Method or process
How the process works
Age is not just a day count divided by 365
Calendar years have different lengths, so completed age changes on an anniversary rather than after a fixed 365-day duration. Months and days in a remainder are also calendar units, not constant lengths.
Leap-day birthdays require a convention in non-leap years. Laws and institutions may recognize February 28 or March 1 for particular purposes; this page’s arithmetic cannot decide that rule.
Compare the concepts
Completed years and exact duration
| Measure | Unit | Best for |
|---|---|---|
| Completed age | Calendar years | Ordinary age statements |
| Exact duration | Days or seconds | Elapsed-time analysis |
Common mistakes
Common mistakes
- Dividing total days by 365.
- Assuming leap-day legal rules are universal.
- Using the current date when another comparison date is required.
Edge cases and limits
Edge cases and limits
- A comparison date before birth is not an ordinary age.
- Historical calendar adoption is outside this proleptic Gregorian model.