Date and time tools that respect the calendar
Age, business days, week numbers, time zones and Unix timestamps. Every one of these runs in your browser and handles the awkward parts, leap years, daylight saving and months of unequal length, so you do not have to.
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IANA time zone aware
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Los Angeles04:15UTC-8
New York07:15UTC-5
London12:15UTC+0
Berlin13:15UTC+1
Mumbai17:45UTC+5:30
Tokyo21:15UTC+9
Sydney23:15UTC+11
All tools
Age CalculatorCalculate exact age from a birthdate including months and daysDate & Time
Date Difference CalculatorCalculate exact days, months, years between two datesDate & Time
Date Format ConverterConvert dates between DD/MM/YYYY, MM-DD-YYYY, ISO 8601, and moreDate & Time
Days Until Date CounterCount down days until any future dateDate & Time
Japanese Date ConverterConvert between Japanese era dates and Gregorian dates both waysDate & Time
Timezone ConverterConvert times across any two time zones with DST supportDate & Time
Unix Timestamp ConverterConvert Unix timestamps to readable dates and backDate & Time
Week Number CalculatorFind the ISO week number for any dateDate & Time
Working Days CalculatorCount working days between two dates excluding weekends and holidaysDate & Time
World ClockSee current time across multiple cities simultaneouslyDate & Time
The hard parts
Four reasons date arithmetic goes wrong
Almost every wrong date on a screen traces back to one of these. Knowing which one you are hitting usually tells you the fix.
01
Months are not a fixed length
Add one month to 31 January and there is no correct answer. Different systems return 28 February, 2 March or an error. Anything that converts months to a fixed number of days is approximating, and the drift compounds.
02
Some days are not 24 hours
On a spring clock change a local day is 23 hours, in autumn it is 25. Divide elapsed milliseconds by 86,400,000 and you silently lose or gain a day twice a year in every zone that observes daylight saving.
03
Inclusive versus exclusive counting
Monday to Friday is four days if you count gaps and five if you count days. Payroll, hotel nights and legal notice periods each pick a different convention, which is why two correct systems disagree by one.
04
Offsets are not time zones
UTC+1 describes a moment. Europe/Berlin describes a place, and carries every past and future rule change with it. Store the offset instead of the zone and your recurring 9am meeting drifts the next time the clocks move.
Reference
Reading an ISO 8601 timestamp
This is the format every one of these tools reads and writes. It sorts correctly as plain text, it cannot be misread between regions, and it is what you should store in a database.
2026–08–11T14:30:00+02:00
2026Four digit calendar year
08Month, always zero padded
11Day of month
14:30:00Time on a 24 hour clock
+02:00Offset from UTC. A Z here means UTC exactly.
| Token | Means | Example |
|---|---|---|
| YYYY | Calendar year, four digits | 2026 |
| MM | Month, 01 to 12 | 08 |
| DD | Day of month, 01 to 31 | 11 |
| HH:mm:ss | Hours, minutes, seconds on a 24 hour clock | 14:30:00 |
| Z | Zulu, meaning UTC with a zero offset | 2026-08-11T12:30:00Z |
| ±hh:mm | Offset from UTC | +05:30 |
| YYYY-Www-D | ISO week date, week and weekday | 2026-W33-2 |
| PnYnMnD | Duration, not a point in time | P1Y2M5D |
One trap worth knowing. In most formatting libraries a lowercase yyyy is the calendar year and an uppercase YYYY is the week based year. They differ for a few days each January, which has caused a wave of production bugs more than once. The full rules live in ISO 8601 and its stricter internet profile RFC 3339.
Pick the right one
What are you actually trying to work out
Several of these tools overlap. Here is the shortest route from a question to the tool that answers it.
How old is someone, exactly
Use the age calculator. It gives years, months and days rather than a decimal, which is what legal, medical and school admission forms ask for.
How far apart are two dates
The date difference calculator reports both a calendar breakdown and an exact day count, because contracts and interest calculations need different ones.
How many working days until a deadline
The working days calculator. You choose which days count as the weekend and paste your own public holidays, so it works outside a Monday to Friday week too.
What time is my call for the other person
The time zone converter for a specific moment, or the world clock if you want several cities visible at once.
What does this number in my log mean
The Unix timestamp converter. Ten digits are seconds, thirteen are milliseconds, and it detects which you pasted.
My supplier quoted delivery in KW34
The week number calculator. KW is Kalenderwoche, the ISO week used across European manufacturing and logistics.
This date is written 03/04/2026, which is it
The date format converter shows the same date in seventeen formats at once, so you can see immediately whether it reads as March or April.
How long until the thing I am waiting for
The days until date counter, which also gives total hours, business days and a progress bar if you set a start date.
Browse more
Where else to look
Every category on ConvertNow, free, and growing as new tools launch.
From the blog
Date and time, explained properly
Longer write ups on the calendar problems these tools solve, from ISO week numbering to scheduling across time zones.
FAQ
Common questions about date and time tools
The things people ask most often before trusting a date calculation.
Do these tools handle daylight saving correctly?
Yes. Offsets come from the IANA time zone database that ships with your browser, so a conversion uses the rule that actually applied on the date you entered. Change the date to January and the answer changes with it, which is why we ask for a date rather than just a time.
What counts as a working day?
Whatever you tell it. The working days calculator lets you choose which weekdays are non working, so it handles a Friday and Saturday weekend or Nepal’s single day weekend just as well as Monday to Friday. Public holidays are pasted in by you, because they vary by country, region and industry.
Why does my age show differently here than on another site?
Almost always inclusive versus exclusive counting. We treat your birth date as day zero, so the elapsed interval is counted rather than the days themselves. A tool that counts both endpoints will report one more day. Neither is wrong, they answer slightly different questions.
What exactly is a Unix timestamp?
The number of seconds since midnight UTC on 1 January 1970. It carries no time zone and no format, which is precisely why systems use it. Ten digits means seconds, thirteen means milliseconds, and the converter detects which you pasted.
Which week numbering do you use?
All three. ISO 8601 with Monday start and week 1 containing the first Thursday, the US convention with Sunday start and week 1 containing 1 January, and a fully custom mode where you set the start day and the fiscal year opening month.
Can I work with dates before 1970?
Yes. Negative timestamps are handled, and the date difference and age calculators accept any valid calendar date including historical ones. Bear in mind that dates before the Gregorian reform in 1582 involve calendar changes that no simple calculator models.
Is anything I type stored or sent anywhere?
No. Every calculation runs in your browser using the JavaScript date engine already on your device. Nothing is transmitted, logged or retained, and closing the tab discards it.
Why do two of your own tools give different day counts?
Because they measure different things deliberately. The date difference calculator reports both a calendar breakdown and an exact day total, since contracts and interest calculations need different ones. The working days calculator excludes whichever days you marked as non working.
How this works
Explore all tools
Your dates never leave the page
Date tools handle birthdays, medical dates, contract terms and personal deadlines. That is the reason all of this runs locally.
Computed in your browserEvery calculation uses the JavaScript date engine already in your device. Nothing is sent to a server, so there is no request log to worry about.
Time zone data from IANAOffsets and daylight saving rules come from the time zone database your browser ships with, which updates as governments change the rules.
Free, with no accountNo sign up, no trial, no export paywall. Close the tab and nothing about your input persists anywhere.