How Date Calculations Work

1. Gregorian calendar rules

Every calculator on this site uses the proleptic Gregorian calendar: the standard calendar in use worldwide, extended backwards and forwards without limit. This means you can calculate the gap between January 1, 100 and March 15, 2025, and the result will be consistent and deterministic.

There are no Julian-calendar corrections, no adjustment for the calendar reform of 1582, and no country-specific calendar rules. Dates are treated purely as Gregorian day counts.

2. Leap years and century rules

A year is a leap year — meaning it has 366 days and February has 29 days — when it satisfies these rules, applied in order:

1. The year is divisible by 400: leap year.
2. The year is divisible by 100: not a leap year.
3. The year is divisible by 4: leap year.
4. Otherwise: not a leap year.

3. Days-between inclusive and exclusive counting

The Days Between Dates calculator supports two counting modes, selectable via a checkbox:

• Exclusive (default): counts the gap between the two dates without counting the start or end day itself. Think of it as counting the nights between two calendar dates.
• Inclusive: adds 1 to the exclusive count, so both the start and end dates are counted. Used when you need to know the total number of calendar days a range spans, including both endpoints.

4. Reversed dates

If you enter an end date that is earlier than the start date, the calculator shows a notice and returns the distance as a positive number. The count is identical in both directions — the gap from May 10 to May 1 is the same as from May 1 to May 10.

This applies to the Days Between, Business Days, and Time Duration calculators. The Add or Subtract calculator is direction-aware and does not treat reversed inputs as an error.

5. Adding years, months, weeks, and days

When you add or subtract multiple units at once, the Add or Subtract calculator applies them in a fixed order: years first, then months, then weeks, then days. This order matches common expectations (anniversaries, deadlines) and avoids order-of-application ambiguity.

The same order applies when subtracting: years, then months, then weeks, then days.

6. Month-end clamping

When adding or subtracting months, the resulting date may fall on a day that does not exist in the target month (for example, February 30). In this case, the calculator clamps the day to the last valid day of the target month.

Clamping only applies to the month step. If you then add days, those days are added from the clamped date, not from the ideal unclamped date. A warning is shown whenever clamping occurs.

7. Business-day assumptions

The Business Days calculator counts weekdays only: Monday through Friday. Saturdays and Sundays are always skipped.

The count is directional when adding days (moving forward or backward in time), and is shown as a positive magnitude when counting between two dates regardless of order.

8. Public-holiday limitation

Public and bank holidays vary by country, region, employer, and year. This calculator does not exclude them. All results assume five working days per week with no holiday adjustments.

If your calculation requires holiday exclusion, add those days manually after getting the weekday count.

9. February 29 birthday conventions

For people born on February 29, the Age Calculator offers two conventions for how to treat their birthday in non-leap years:

• February 28 (default): the birthday is observed on the last day of February. This is the convention used in most of Europe, Australia, and Canada.
• March 1: the birthday is observed on the first day of March, the day after the normal end of February. This is the convention used in some legal systems including the UK and New Zealand.

The policy selector only appears when a February 29 birth date is entered. In leap years, the anniversary is always February 29 regardless of the chosen policy.

10. Chronological versus legal age

The Age Calculator computes chronological age: the number of complete years, months, and days elapsed from the birth date to the as-of date. This is the age you would state informally ("I'm 34 years old").

Legal age — for voting, contracts, retirement, or other statutory purposes — can differ by jurisdiction in how it counts the birthday itself, partial years, and February 29 anniversaries. Always confirm legal age with the relevant authority if it matters for a formal decision.

11. Local-clock time duration

The Time Duration Calculator uses the times exactly as you enter them, treating them as local clock times on their respective dates. The result is the arithmetic difference between those two date-time pairs, counted in days, hours, minutes, and seconds.

A disclosure note on the result reminds you that the calculation is based on your entered clock values, not on a timezone-aware timestamp.

12. Time-zone and daylight-saving exclusion

None of the calculators on this site adjust for time zones or daylight-saving time transitions. If your start and end times cross a DST boundary — for example, a clocks-back night in October — the calculator will not add or remove an hour automatically.

For time-zone-aware calculations (flight times, cross-border meetings, scheduled events), use a tool that accepts explicit IANA time-zone identifiers.

13. Privacy: calculations happen in your browser

Every calculation on this site runs entirely in your browser. Your input dates, times, birth dates, and results are never transmitted to a server. No date or result values are stored beyond the current browser session, and no date or result values are included in analytics events.

Session storage is used only to restore your last inputs when you navigate between calculators in the same tab. Closing or refreshing the tab clears all stored values.