Date of birth calculator

Use the mode that matches the evidence you actually have. Adding guessed months or days makes the output look more precise without making the source record more precise.

Years, months and days mode is for a full age statement such as 25 years, 4 months and 6 days on a known reference date. Completed years only is for ordinary statements such as 25 years old. Total calendar days is for an exact day-count convention.

The February 29 anniversary selector appears for the two age-based modes because it changes how forward ages are checked. Total calendar days mode does not use that selector because subtracting a day count is a different operation.

Subtraction can erase information near month ends. Instead of subtracting a displayed age once and trusting that single answer, the engine checks every plausible birth date in the relevant year span.

Each candidate is sent through the same forward age calculation used by the age calculator. If the years, months and days match the input, the candidate is kept; if not, it is discarded.

This is why some inputs produce several matches and some produce no match. A no-match result is not just an error message; it can reveal an impossible combination such as a day count or month/day pair that the forward rule never produces for that reference date.

Use reference date 2023-02-28 and age 0 years, 1 month, 0 days. Under the forward month-end rule, January 28, January 29, January 30 and January 31 can all reach February 28 after one completed month in 2023.

If a tool simply subtracts one month from February 28, it may return only January 28. That answer is not wrong as one possible date, but it is incomplete for this forward-checking rule.

This is the main reason the page can show a candidate list. It protects the user from a false exact date when the calendar operation has collapsed several starts into one observed age.

For a completed age of 25 on 2026-06-15, the latest ordinary birthday is 2001-06-15: that person turns 25 on the reference date. The earliest is 2000-06-16: that person will turn 26 the following day. Someone born on 2000-06-15 is already 26 and is excluded. This is why simply subtracting 25 from the reference year gives only one edge of the answer, not a reconstructed birthday.

A years-only range need not contain exactly 365 dates. For one completed year on 2021-02-28, the February 28 rule includes 2019-03-01 through 2020-02-29, or 366 candidates. The March 1 rule excludes the leap-day birthday at the upper end, leaving 365. The count comes from matching actual dates under the selected rule, not from assuming a standard-length year.

Total calendar days is a different kind of evidence. With reference 2024-03-01, counts 0, 1 and 2 produce 2024-03-01, 2024-02-29 and 2024-02-28 respectively. Each result is unique because moving an integer number of civil days back from a known date has one endpoint. The February 29 anniversary selector is not needed for that operation; the leap day is still counted as a calendar date.

Check the original counting convention before entering a total. If a source calls the birth date day 1 and the next date day 2, its inclusive label of 2 corresponds to a one-day difference here. Subtract one only when the source explicitly uses that convention. Do not convert a rounded age, a number of completed weeks or elapsed hours into an exact total-day input and expect the uncertainty to disappear.

A precise-looking number is only as precise as the record behind it. A form field that says age 25 may have been entered after a birthday, before a birthday or from memory; the calculator can only model the statement you enter.

If the record says about 25, entering exactly 25 does not carry the word about into the math. Keep that uncertainty in your notes and avoid replacing an authoritative record with a reconstructed date.

When two records disagree, calculate their implications separately using each record's own date. Their overlapping candidate dates may be useful leads, but overlap is not proof that either age was accurately recorded. Do not average two reconstructed endpoints to manufacture a birthday. Look for a better source, a transcription error, a different calendar or an anniversary convention before narrowing the evidence.

The US Naval Observatory leap-year rule states the Gregorian exception for century years: divisible by 100 is not enough unless the year is also divisible by 400. That is why 1900 and 2100 are not leap years while 2000 is.

For February 29 births, the non-leap-year anniversary setting can change whether a candidate has already completed a year near the end of February or the start of March.

When the original convention is unknown, compare both February 28 and March 1. A one-day boundary difference can change the reconstructed range even when the reference date and age text are unchanged.

The calendar system behind an old record also matters. This calculator consistently applies Gregorian rules to every supported year; it does not identify regional calendar adoption or translate a Julian label. Preserve the original date text and any documented conversion before doing the reconstruction. Otherwise a result can be internally consistent while answering a question about the wrong civil date.

A validation warning and a no-match result mean different things. Blank or fractional age fields are not valid whole-number inputs; months must be 0-11 and days 0-31, even if a source writes a duration as 14 months. Do not silently reduce that source to one year and two months without knowing its age convention. A validly entered combination can still have no match under the forward rule.

For example, 0 years, 0 months and 31 days on 2023-03-01 has no match here. A January start far enough back to accumulate 31 days already completes a clamped calendar month by the reference. The warning is asking you to inspect the age statement, not to pick the nearest plausible birthday. In total-days mode, subtraction before 0001-01-01 likewise has no supported result.

Copy and Download keep the mode, reference date, result, count and relevant inputs. When full-age mode has several candidates, its exported summary also includes the ISO candidate list. Review that list, not only the earliest and latest dates, before discussing a possible birthday. The file clocktools-birth-date.txt is a labeled text summary, not a certificate or an automatic database update.

Share page sends only the link, with query string and fragment removed. It does not restore your source record or current calculation for another person, and there is no saved history. If clipboard or download access is blocked, manually record the candidate list and its assumptions. A failed guide load can be retried without resetting the calculator; it does not imply that the mathematical result failed.