How date answers are calculated and interpreted.

Age in Days is a browser-first utility. This page explains the practical assumptions behind each mode so results can be interpreted with confidence, not treated as hidden policy outcomes.

Core model

All tools use one shared calendar-date utility. Inputs must be literal valid YYYY-MM-DD dates: format, month, day range, and reconstructed components are checked before any calculation. Impossible inputs such as April 31 or February 29 in 2026 are rejected rather than silently moved into another month.

Calendar assumptions

Dates are calendar dates, not time stamps.
Date inputs are interpreted in the browser’s local calendar, then compared through a UTC day serial made from those calendar parts; daylight-saving hour changes do not change the day count.
Leap days are included through actual calendar rules.
Month breaks depend on calendar boundaries, not fixed 30-day blocks.

Counting rules

Age, countdown, and ordinary date-span results count calendar boundaries: the start date is day zero, so the same date is zero days and the next date is one day. Date-span results also state the inclusive alternative, which adds one only when both listed endpoints are meant to count. Working-day mode intentionally includes both selected endpoints while filtering Saturday and Sunday.

Birthdays, milestones, and approximations

The next-birthday tool uses February 28 as its informational display convention for a February 29 birth in a non-leap year; this is not legal advice. A days-lived milestone is the birth date plus the selected number of calendar days, with birth date as day zero. Hours and minutes are shown only as day-total × 24 and × 1,440 approximations because a birth time and timezone are not supplied.

Testing and updates

Regression tests cover invalid dates, Gregorian century exceptions, leap days, same/adjacent dates, month/year boundaries, daylight-saving spans, inclusive counting, next birthdays, and day milestones. The calculation contract is reviewed when date logic changes; confirmed corrections receive a dated update.

Reverse ranges

For two-date spans, the tool normalizes input order so the span can still be interpreted coherently. The returned total remains a numeric absolute span, while internal state tracks direction for narrative phrasing.

Working days

The working-day mode excludes Saturday and Sunday from the counted range. It does not apply region-specific holidays, office closures, or custom shifts automatically.

What changes between modes

Age mode focuses on elapsed age units.
Date span mode uses the same base range math with year/month/day decomposition.
Countdown mode labels targets before or after today differently.
Working days mode uses weekday filtering.

Why the method page exists

Users need a single place to verify assumptions, especially for edge cases. This page keeps the interpretation contract visible before deeper guide reading.