Calculate age, birthdays, elapsed time, workdays, calendar dates and time zone conversions.
Browse 19 date and time tools plus a related sleep planner. Check the calendar and time zone assumptions on each tool before using its result.
Featured date and time tools
8 toolsCommon tools for deadlines, durations, age and date comparisons.
Dates, age and calendars
6 toolsWork with birthdays, week numbers and calendar conventions.
Time zones, clocks and timestamps
3 toolsCalculate elapsed hours and convert between clocks or timestamps.
Cultural and astronomical dates
2 toolsExplore Hijri dates and moon phases with the method explained.
Choose and check a date tool
Start with the question
Choose age for a birthdate, date difference for two calendar dates, or business days when weekends and selected holidays matter.
Confirm the rules
Check whether endpoints are counted, how month ends and leap days are treated, and which holidays the tool includes.
Check the time zone
For an event at a specific instant, enter the intended location and time. Named time zones can change offset with daylight saving rules.
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
Quick answers about calendar rules, durations and time zones.
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 calculator categories
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