Date and time tools handle a kind of arithmetic that looks simple and is quietly full of traps. Months have different lengths, some years carry an extra day, clocks run on two competing formats, and the world is divided into time zones that do not line up neatly. A single overlooked rule can throw a result off by a day, an hour, or a whole year. This guide is a map of the date and time tools on this site, grouped by the job each one does.
In this guide
Why date and time maths goes wrong
The calendar is not a regular system. Months range from 28 to 31 days. Most years have 365 days, but some have 366. The leap year rule even has an exception inside its exception: a year divisible by 4 is a leap year, except a century year, which counts only if it is also divisible by 400. The year 2000 was a leap year, but 1900 was not.
Clocks add their own confusion with two formats in daily use, and time zones add another layer on top. Any calculation that crosses these boundaries can drift if one rule is missed. A tool applies the rules the same way every time, which is the point of using one.
Calendar tools: dates, years and ages
The calendar tools work on dates and the years that hold them. The leap year checker tells you whether a given year has 366 days, and the common year checker confirms the reverse, both applying the divisible-by-400 rule so you do not have to.
The calendar date analyzer breaks a single date down into its details, such as the day of the week it falls on. The age calculator works out an exact age in years, months, and days from a birth date, handling the uneven month lengths automatically, and the date at percent of year tool finds the calendar date that sits a given percentage of the way through a year.
Clock tools: time of day
The clock tools work on times of day rather than dates. The clock time analyzer breaks a time down into its parts. The seconds since midnight and seconds till midnight tools convert a time of day into a plain count of seconds, which is how many systems store time internally.
For something more specific, the clock hand angle calculator works out the angle between the hour and minute hands at a given time. At three o’clock, for instance, the hands sit exactly 90 degrees apart.
Time format conversion
The 12-hour and 24-hour formats cause real, everyday confusion. The 12-hour format runs 1 to 12 twice a day with AM and PM to mark which half. The 24-hour format runs 00 to 23 once, with no AM or PM. The 24-hour time 15:00 is 3:00 PM, midnight is 00:00, and noon is 12:00.
The point where people slip is the boundary around midnight and noon, where the two systems do not map in an obvious way. The 12-hour to 24-hour converter and the 24-hour to 12-hour converter handle both directions without the guesswork.
Time zones
Time zones are the hardest part of date and time work. Zones differ from each other by whole hours, and in some places by half hours, and many regions shift their clocks seasonally for daylight saving. Working out what time a meeting falls in another country is a calculation people get wrong constantly.
The time zone difference calculator works out the gap between two zones, which is the figure you need to schedule a call across regions without waking someone at three in the morning. The full set of these tools lives in the time tools category, and each one runs in your browser with no signup.
Free date and time tools used in this guide
Frequently asked questions
How do I know if a year is a leap year?
A year is a leap year if it is divisible by 4, except for century years, which count only if they are also divisible by 400. 2024 is a leap year, and 2100 will not be.
What is the difference between 12-hour and 24-hour time?
The 12-hour format runs 1 to 12 twice a day with AM and PM. The 24-hour format runs 00 to 23 once. The 24-hour time 15:00 is the same as 3:00 PM.
How do I calculate someone’s exact age?
The age calculator works out years, months, and days from a birth date, handling the uneven lengths of the months for you.
Why is calculating across time zones difficult?
Zones differ by whole and sometimes half hours, and daylight saving shifts many of them seasonally, so the gap between two places is not always a fixed number.
Are these tools free?
Yes. Every date and time tool on the site is free, runs in your browser, and needs no account.