Age Calculator

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Calendar with clock representing age from birth to a target date.

Age is the elapsed time between a birth date and a chosen moment, expressed in everyday calendar terms. It helps you read milestones with clarity so planning and comparisons are simple.

You supply a date of birth and a date to measure against, and you may add the exact time for both moments for hour level precision. Results arrive as calendar years and months and days, along with a decimal years view for quick arithmetic.

The tool also reports the next birthday date and how many days remain, plus how far you are through the current birthday year. You can set a personal lifespan target to see life progress as a percentage and you will also see weekday of birth and two common zodiac labels.

For a quick example, a birth on 1 January 1990 measured at 13 October 2025 reads as 35 years 9 months 12 days. The next birthday is 1 January 2026 and the countdown shows 80 days remaining with about 78.1 percent of the year passed.

For reliable comparison, keep the same time choice when repeating a check and prefer whole days when the hour is unknown. If you were born on 29 February, a non leap year celebration can be set to 28 February or 1 March.

Technical Details:

The underlying quantities are two civil moments on the calendar, the birth moment and the target moment. Differences are computed in calendar aware units so month length and leap years are respected, and totals are also available as seconds, minutes, and hours for straightforward aggregation.

Age in years and months and days is presented as complete years first, then remaining whole months, then remaining whole days. A continuous “age in years” value is also produced for arithmetic and trend tracking, rounded to a selected number of fractional digits.

Progress through the current birthday year is measured from the last birthday to the next birthday. The percentage passed is the share of days already elapsed in that span, and the remaining percentage is the complement. A personal life progress percentage is available when a lifespan in years is provided.

For births on 29 February, the next birthday in non leap years follows the selected convention. “Feb 28” uses 28 February and “Mar 1” uses 1 March. The default “Nearest” behaves like 28 February in this implementation.

\begin{matrix} Y_dec & = & yearDiff (T, B) \\ p_passed & = & 100 \times \frac{D}{_} \\ p_remain & = & 100 - p_passed \\ p_life & = & clip (0, 100, 100 \times \frac{Y}{_}) \end{matrix}

Symbols and units
Symbol	Meaning	Unit/Datatype	Source
$B$	Birth moment	date or date‑time	Input
$T$	Target moment	date or date‑time	Input
$Y$	Whole years between B and T	year	Derived
$M$	Whole months after full years	month	Derived
$D$	Whole days after months	day	Derived
$D_passed$	Days since last birthday	day	Derived
$D_span$	Total days in current birthday year	day	Derived
$Y_dec$	Age as a continuous year fraction	year	Derived
$L$	Lifespan target	year	Input

Worked example

Inputs: birth 1990‑01‑01, target 2025‑10‑13, time not included, leap convention “Nearest”.

\begin{matrix} Age & = & 35 y 9 m 12 d \\ NextBirthday & = & 2026 ‑ 01 ‑ 01 \\ DaysTo & = & 80 days \\ p_passed & \approx & 78.1 % \end{matrix}

Western zodiac Capricorn and Chinese zodiac Horse follow the date based lookups in the logic.

Units, precision, and rounding

Seconds, minutes, and hours totals are truncated toward zero. The year progress percentages are rounded to one decimal place. The decimal age is rounded to the selected number of digits from 0 to 12.

Validation and bounds

Validation rules
Field	Type	Min	Max	Step/Pattern	Error Text
Date of birth	date	—	—	—	“Enter valid dates.”
Age at date	date	—	—	—	“Enter valid dates.”
Include time of day	boolean	—	—	—	—
Time of birth	time	—	—	`^\d{2}:\d{2}$`	—
Target time	time	—	—	`^\d{2}:\d{2}$`	—
Leap‑day convention	enum	—	—	nearest \| feb28 \| mar1	—
Decimal age digits	number	0	12	step 1	—
Lifespan (years)	number	0	—	step 1	—
Birth after target	constraint	—	—	—	“Target date must be on or after the birth date.”

I/O formats

Input and output formats
Input	Accepted Families	Output	Encoding/Precision	Rounding
Dates and times	YYYY‑MM‑DD; HH:mm	Readable age; totals; progress	Human readable text	Totals truncated; percents to 0.1
Exportables	—	CSV; JSON payload	JSON keys mirror input names	Decimal age fixed digits 0–12

Time and calendrics

Dates and times are interpreted in the local system time zone. Day boundaries use local midnight. Leap years follow the Gregorian rule and the non leap handling for 29 February follows the chosen convention.

Privacy and compliance

No data is transmitted or stored server‑side. Entered dates remain in your session only.

Assumptions and limitations

Local time zone is assumed for both moments.
Decimal age uses calendar aware differences, not a fixed 365 day year.
Percent of year is based on whole days between birthdays.
Daylight saving transitions may shift apparent hour totals.
Month counts depend on varying month lengths.
Very large lifespan values compress the life progress scale.
Invalid times default to 00:00 when time is included.
Heads‑up “Nearest” leap convention maps to 28 February in this logic.

Edge cases and error sources

Empty dates produce a validation error.
Target before birth triggers a blocking error.
Non matching time strings are ignored and treated as 00:00.
Cross midnight checks can change day counts at the boundary.
Switching the include time option changes totals by up to one day.
Setting decimal digits above 12 is clamped to 12.
Lifespan set to zero disables life progress.
February spans vary by year and affect day totals.
System clock or time zone changes alter results.
Copy or download reflects the current tab’s data definitions.

How to use:

Age measurement compares a birth date to a target moment and returns calendar totals plus a decimal years view.

Enter the birth date required.
Enter the date to measure against required.
Switch “Include time of day” if hour precision is needed.
Provide birth time and target time when time is included.
Choose a leap day convention if born on 29 February.
Set decimal digits from 0 to 12 if you need a specific precision.
Optionally add a lifespan in years to see life progress.

Example: birth 1 Jan 1990, target 13 Oct 2025, time off → 35 y 9 m 12 d, next birthday 1 Jan 2026, 80 days to go.

If hours are unknown, leave time off for stable day counts.

FAQ

Is my data stored?

No. Processing is client only and nothing is sent to a server or saved beyond your session.

Dates and times stay on your device.

How accurate is decimal age?

It uses calendar aware year differences, reflecting real month lengths and leap years, then rounds to your chosen number of digits.

What if I was born on Feb 29?

Choose 28 February or 1 March for non leap years. The default option uses 28 February in this implementation.

Can I include hours and minutes?

Yes. Turn on the time option and provide birth and target times for hour level precision.

Which units are shown?

You get years, months, weeks, days, hours, minutes, seconds, a decimal years value, and progress percentages.

Does it work offline?

Yes, once the page is loaded. All computation runs on your device.

Is there a cost or license?

No explicit license is included in the provided files. Usage terms may depend on the site where this tool appears.

What does a borderline result mean?

Values near a day boundary can flip when time is included. Keep time off if only full days matter.

Glossary

Birth moment: The starting date or date and time.
Target moment: The date or date and time used to measure age.
Decimal age: Age as a continuous fraction of a year.
Year progress: Share of days passed between birthdays.
Lifespan: Personal target in years for life progress.
Leap day convention: Rule for 29 February birthdays in non leap years.