Working out durations, deadlines and calendar arithmetic. Leap years, month lengths and daylight saving are handled correctly — which is exactly where doing it by hand goes wrong.
How old is someone → Age Calculator. Days between two dates → Date Difference. A deadline N days out → Add or Subtract Date. Working days only → Business Days. Across time zones → Time Zone Converter.
Featured
- Add or Subtract Date Calculatorthe date a number of days, weeks, months or years from another.
- Age Calculatorexact age in years, months and days, plus total days lived and the next birthday.
- Date Difference Calculatorthe span between two dates, with and without the end date counted.
- Day of the Week Calculatorwhich weekday any date falls on, past or future.
- Business Days Calculatorworking days between two dates, excluding weekends and holidays you specify.
- Countdown Calculatortime remaining to a date, live to the second.
- Time Duration Calculatoradds and subtracts durations in hours, minutes and seconds.
- Time Zone Convertera time across zones, with daylight saving applied for the actual date.
Dates, age & calendars
- Age Difference Calculatorthe gap between two birthdates, in years, months and days.
- Generation Calculatorwhich generation a birth year falls in, with the boundary years each source uses.
- Julian Date Calculatorconverts between calendar dates and Julian day numbers, used in astronomy and some data systems.
- Leap Year Calculatorwhether a year is a leap year, and the next and previous ones.
- Week Number Calculatorthe ISO week number for a date, and the dates a given week covers.
Time zones, clocks & timestamps
- Hours Calculatortotal hours and minutes between two times, across midnight if needed.
- Time Card Calculatorweekly hours from daily clock-in and clock-out times, with breaks deducted and overtime split out.
- Unix Timestamp Converterepoch seconds to a readable date and back, in local time or UTC.
Cultural & astronomical dates
- Hijri–Gregorian Converterconverts between the Islamic lunar calendar and the Gregorian calendar in both directions.
- Moon Phase Calculatorthe lunar phase on any date, with illumination percentage.
Related tools
Useful cross-category tools connected to this topic.
- Sleep Calculatorbedtimes and wake times aligned to complete sleep cycles.
Why date arithmetic goes wrong by hand
Adding thirty days to a date sounds trivial and produces errors constantly, because the calendar is not regular. Months run 28 to 31 days, February changes length in a pattern with three separate rules, and daylight saving makes two days a year 23 or 25 hours long.
| The trap | What actually happens |
|---|---|
| "A month from 31 January" | There is no 31 February. Different systems return 28 February, 2 March, or an error — and payroll, subscriptions and contracts each handle it differently |
| Leap years every four | Not quite. Century years are skipped unless divisible by 400 — 1900 was not a leap year, 2000 was |
| Counting days between dates | Inclusive or exclusive? Monday to Friday is 4 days apart but 5 working days. Both answers are right for different questions |
| Converting a time across zones | The offset depends on the date, not just the zone — the gap between London and New York is 5 hours most of the year and 4 for two weeks in spring |
Every calculator here applies these rules rather than approximating them, and states which convention it used when there is a choice.
Frequently asked questions
Does the age calculator count leap years?
Yes — it works from actual calendar dates rather than multiplying by 365.25, so the day count is exact. For anyone born on 29 February it reports the birthday as 28 February in common years, which is the convention most jurisdictions use for legal age.
Should the end date be included when counting days?
It depends what you are counting, which is why the Date Difference Calculator shows both. For a duration — how long between two events — exclude it. For occupancy or a rental period, include it: a hotel counts nights, but a car hire counts days. Contracts usually specify; when one does not, ask rather than assume.
How does the time zone converter handle daylight saving?
It applies the rules in force for the specific date you enter, not today's offset. That matters when scheduling ahead: the gap between two cities can change between now and the meeting, because countries switch on different dates and some do not switch at all. Converting a future meeting time with today's offset is a classic scheduling error.
What is a Unix timestamp?
The number of seconds since 1 January 1970 UTC — the standard way computers store time internally, because a single integer avoids every timezone and calendar complication. You will meet it in databases, log files and APIs. The converter handles both seconds and milliseconds, since JavaScript uses milliseconds and most other systems use seconds.
Why does the Hijri date differ by a day from what I expected?
Because the Islamic calendar can be reckoned two ways. Tabular calculation follows a fixed arithmetic rule, while observational reckoning depends on the actual sighting of the crescent moon, which varies by location and conditions. A one-day difference between a calculated date and a locally announced one is normal. For religious observance, follow your local authority rather than any calculator.
Can the business days calculator handle my country's holidays?
You enter them yourself, which is deliberate. Public holidays differ by country, region and sometimes employer, and several move each year with the lunar or ecclesiastical calendar. A built-in list would be wrong for most users most of the time; entering your own is a minute of work and gives an answer that is actually correct for you.
Other categories
- Unit Converterstime, length, weight and temperature
- Business Calculatorspayroll, PTO and working time
- Lifestyle Calculatorstravel, cooking and everyday tools
- Math Calculatorsarithmetic, statistics and geometry