Age calculator
Calculate age at a chosen date, birthday progress and days until the next anniversary.
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Dates & time
Dates and times, calculated the way people actually count them.
Business days calculator
Count working days with selectable weekdays, endpoint rules and your own holiday dates.
Date add / subtract
Move a date by years, months, weeks and days with clear month-end adjustments.
Date difference
Count calendar days between two dates and explore the range on a calendar.
Duration calculator
Add or subtract precise durations and see the result in hours, days and milliseconds.
ISO week calculator
Convert a calendar date to an ISO week date, or find the date for a numbered week.
Timezone converter
Compare one UTC instant across IANA timezones with side-by-side clocks and date changes.
Unix timestamp converter
Convert exact Unix seconds or milliseconds to UTC and back with a live instant preview.
Calculate ages, business days, calendar differences, durations, ISO weeks, and Unix timestamps, and compare one instant across time zones. Month-end and leap-year cases are handled explicitly.
Everything uses the Gregorian calendar and runs locally; no time-zone service is queried.
Calendar maths vs. elapsed time
Date questions come in two kinds. Calendar questions (how old is someone, when is 3 months from today, how many working days are in this project) depend on months of different lengths, leap years, and weekends. The age calculator, date difference, date add/subtract, business days, and ISO week tools answer them using calendar rules, independent of time zones.
Elapsed-time questions, such as how long a task took or what time it is in Tokyo when a server logs an event, depend on exact instants. The duration calculator adds and subtracts precise durations, the Unix timestamp converter translates between epoch seconds and readable UTC, and the time zone converter shows one instant across several zones with their daylight-saving rules applied.
Rules made explicit
Many date bugs come from unstated assumptions. Is the end date included? What is January 31 plus one month? Does a February 29 birthday fall on February 28 or March 1 in other years? Is week 1 the week containing January 1 or January 4? Each tool states its convention and, where there is a genuine choice, lets you pick it.
All tools use the Gregorian calendar and ISO 8601 formats (YYYY-MM-DD), which sort correctly and are unambiguous across countries. Unlike 03/04/2026, which means different dates in the US and in Europe.