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The date is required; the time field is used only when time precision is on.
Use Today to restore the current local date and minute.
Fill the current local date{{ include_time ? ' and minute' : '' }}.
Leave off for date-only records; guessed times create false precision.
Calendar age uses completed anniversaries and real month lengths.
Match the convention used by the record or organization relying on the result.
Gregorian mean fits ordinary civil-calendar comparisons.
Classic includes familiar birthday ages; interval patterns use evenly spaced birthdays.
Choose a whole interval from 1 to 20 years.
years
Choose 1–60 years.
{{ milestone_horizon_years }} years
Choose 0–120 days; zero marks only the anniversary timestamp.
{{ birthday_alert_days }} days
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Standard is compact; detailed adds decimal places to elapsed and cycle values.
MeasureValueMeaningCopy
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CheckpointDateDays awayPlanning noteCopy
{{ row.label }}{{ row.date }}{{ row.daysAway }}{{ row.note }}
Use the calendar result for birthday wording

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Keep elapsed totals tied to their assumptions

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Treat milestone rows as reminders

Birthday checkpoints do not establish legal age, school cutoffs, benefits, medical bands, or account eligibility. Verify the governing rule and its own as-of date.

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Age can mean a count of completed birthdays or the exact duration between two timestamps. Those answers agree on the number of whole years in ordinary conversation, but they serve different purposes. Calendar wording follows anniversaries and real month lengths; elapsed time treats the interval as a continuous count of days, hours, and minutes.

ResultBest useMain assumption
Calendar ageOrdinary wording such as 36 years, 7 months, and 11 days.Completed anniversaries and calendar-month boundaries.
Elapsed totalsComparing exact duration in days, hours, or minutes.Every modeled day has 24 hours and every hour has 60 minutes.
Decimal yearsExpressing elapsed days as a fraction of a selected average year.The chosen year-length divisor, not birthday anniversaries.
Birthday planFinding the next anniversary and future milestone dates.The same leap-day rule and as-of date used for the age result.

A person born on January 31 does not gain a full calendar month on every 31st because some months end earlier. Someone born on February 29 also needs an anniversary convention in non-leap years. February 28 and March 1 are both used in different administrative or personal contexts, and choosing between them can change the completed-year result at that boundary.

Time of day adds precision only when both timestamps are known. Guessing a birth time can make an answer look more exact without making it more accurate. For date-only records, completed calendar days are usually the clearer result.

Decision limit: A calculated birthday does not establish legal majority, school placement, retirement eligibility, benefits, medical bands, or account access. Those decisions may define their own cutoff date, timezone, and February 29 rule.

How to Use This Tool:

Use the same record conventions as the decision or comparison that needs the age.

  1. Enter the Birth timestamp and As-of timestamp. The as-of value must be the same as or later than the birth value.
  2. Turn Time precision on only when both times are known. When it is off, the calculation uses the two civil dates without guessed times.
  3. Choose the Feb 29 anniversary rule and the Decimal-year basis. The year basis changes decimal years only; calendar age and birthday dates stay the same.
  4. Select a milestone pattern and planning horizon if future birthday checkpoints matter. Read Age breakdown for the current result and Birthday plan for upcoming dates.

Interpreting Results:

Calendar age is the natural birthday-based answer. Elapsed days and Decimal years are duration measures, so they should not replace the calendar result in a rule that asks how many birthdays have been completed.

  • Completed calendar months equals completed years multiplied by 12 plus the months after the most recent anniversary.
  • Days until birthday is rounded up to the next whole day. The birthday is marked as today throughout the matching civil date.
  • Birthday-year progress compares the as-of timestamp with the interval between the previous and next chosen anniversary.
  • Milestone dates are reminders only. Verify any externally governed cutoff against its own rules and timezone.

Technical Details:

The date model uses the proleptic Gregorian calendar for years 1 through 9999. A leap year is divisible by 4, except a century year must also be divisible by 400. Civil dates are converted to a timezone-neutral minute coordinate, so daylight-saving transitions and local UTC offsets do not alter the interval.

Formula Core

Elapsed totals come from the difference between the as-of and birth coordinates. With Δt measured in milliseconds:

Delapsed=tas-of−tbirth86400000

Decimal years divide elapsed days by the selected year-length convention:

Ydecimal=DelapsedBdays
BasisDaysInterpretation
Gregorian mean365.2425Average civil-calendar year across the 400-year Gregorian cycle.
Tropical year365.24219Approximate seasonal-year convention.
Julian year365.25Fixed 365.25-day duration convention.

Rule Core

Calendar age is decomposed in order rather than derived from a fixed days-per-month factor:

  1. Find the latest whole-year anniversary that is not after the as-of timestamp.
  2. Advance from that anniversary by as many whole calendar months as fit, clamping a day such as the 31st to the destination month's last valid day.
  3. Convert the remaining interval to whole days and, when time precision is on, whole hours and minutes.
Selected ruleNon-leap anniversaryBoundary effect
February 28February 28The completed year is reached on February 28.
March 1March 1February 28 remains in the preceding completed year.

Future milestone dates must be later than the as-of timestamp and no later than the selected horizon. At most eight upcoming milestones are returned. Calculation values retain full precision; the display setting changes visible decimal places only.

Accuracy Notes:

The result is deterministic for the entered civil timestamps and selected conventions, but those conventions may not match an external authority.

  • No timezone, daylight-saving transition, leap second, or historical calendar change is modeled. Every elapsed day is exactly 24 hours.
  • Dates before the historical adoption of the Gregorian calendar are projected backward using Gregorian rules.
  • February 29 treatment differs across laws and organizations. Match the rule used by the record keeper rather than assuming one universal anniversary.
  • Decimal years are approximate duration expressions. Changing the divisor changes that value but not completed calendar years, months, days, or birthdays.

Worked Examples:

A February 29 birthday in 2026

For a birth date of February 29, 2000 and an as-of date of February 28, 2026, the February 28 rule gives 26 completed calendar years. The March 1 rule gives 25 years because the 2026 anniversary has not arrived. Elapsed days are unchanged; only the calendar anniversary convention differs.

References: