Age calculator

For birth date 2023-01-31 and reference date 2023-03-01, no complete year has passed, so January 31 remains the year anchor. The tool checks one calendar month from that anchor. It lands on February 28 in 2023 because February has no 31st day. This adjustment is called clamping: keep the requested month, but use its last valid day when the original day does not exist.

The next day, March 1, is then one remaining civil day after the clamped monthly anchor. The result is not 29 divided by an average month; it is zero years, one month and one day under this tool's month-end convention.

This example is useful when comparing calculators. If another calculator reports zero months and 29 days, it may be using a duration convention instead of anniversary-and-month arithmetic. Neither number is self-explanatory unless the rule is stated.

Every month candidate is measured from the same year anchor. For January 31 to March 31, adding two months directly gives March 31, so the age is two months and zero days. Repeatedly adding one month would go January 31 to February 28 to March 28 and introduce a three-day drift. The independent anchors avoid that drift; they do not turn all months into equal durations.

The large years, months and days numbers are the calendar age on the selected reference date. They are the safest row to quote when someone asks how old a person is on a date.

Total calendar days is an exclusive day difference: it is zero on the birth date, one on the next date, and so on. Completed weeks is that day count divided by seven and rounded down; completed months is years times twelve plus the displayed month count.

Next birthday is calculated using the same February 29 rule selected in the form. If the reference date is the birthday, the days remaining value is zero because the next anniversary has already arrived on that date.

Enable the time option only when both clock times are known and the device time zone is appropriate for both endpoints. The form has birth and reference times, but no separate birth-place or time-zone selector. A recorded 09:00 in another country is not automatically interpreted in that country's zone. Leave times off when you only need civil age, or use a zone-aware conversion outside this tool before making an elapsed-time comparison.

With a device set to America/New_York, noon on 2024-03-09 to noon on 2024-03-10 spans one civil day but 23 elapsed hours; noon on 2024-11-02 to noon on 2024-11-03 spans 25 hours. These are fixed examples of offset changes, not results for your present location. The hour row is displayed with up to two decimal places. That presentation is not evidence that an uncertain historical birth time is known precisely.

A skipped local time does not correspond to an instant, and a repeated local time can correspond to two. The tool rejects both rather than silently selecting an offset. It also rejects a reference instant before the birth instant, including reversed clock times on the same date. Calendar age can remain visible when hours fail, but result copy/download are disabled until the times are corrected or the time option is turned off.

For a February 29 birth date, the selected rule affects completed years, the next birthday and any result exported from the page. Keep the rule with the copied or downloaded summary so the number can be interpreted later.

February 28 treats the non-leap-year anniversary as the last day of February. March 1 treats it as the next calendar date after February 28. The tool does not decide which rule an institution should use.

When you are checking a family note or a form that does not say which convention was used, run both rules and record the difference. If the difference matters, consult the policy or source record rather than treating the calculator as an authority.

For birth 2020-02-29 and reference 2021-02-28, the February 28 rule gives one completed year; the March 1 rule gives zero completed years, 11 months and 30 days. On 2021-03-01, the March 1 rule gives exactly one year, while the February 28 rule gives one year and one day. The total civil-day difference is unchanged by either choice: only the anniversary-based decomposition changes.

The date inputs are Gregorian civil dates, not a universal decoder for every historical calendar. A birth recorded under a Julian, lunar, religious or local civil calendar may need conversion before it is entered.

The US Naval Observatory leap-year rule is enough to explain why simple every-four-years shortcuts fail for 1700, 1800, 1900 and 2100, but it does not tell you which calendar a handwritten register used.

For genealogy or archival work, keep the original wording, the calendar system if known, the converted date if you use one, and the calculator rule. The result is easier to audit when the conversion assumptions are not hidden.

Copy result keeps a labeled summary; Download offers the same information in clocktools-age.txt. Both include the birth and reference dates, age components, totals and selected anniversary rule. A valid time calculation also includes both clock times and the device zone. Keep that context when comparing calculations made on different days: an age number without a reference date quickly becomes ambiguous.

Share page opens supported native sharing or copies the page link. It deliberately strips the query string and fragment and does not attach your dates or summary. To let someone check your calculation, send a separately reviewed summary or fictional reproduction rather than expecting the shared link to restore it. The calculator does not create a saved input history, so download a needed result before leaving or resetting the page.

If Copy is blocked, select the displayed information manually. If Download reports that it has started, check the browser's downloads before assuming a file was saved. For an unavailable hours result, correct the times or turn the time option off to export the date-only calculation. If the guide fails to load, its retry action leaves your working inputs alone; a guide-loading message is not an arithmetic error.