Date and time calculators
DateTimeHQ is a set of free calculators for working with dates. Find an exact age, count the days between two dates, see how long until or since an event, or add and subtract days from any date. Every tool runs in your browser, so results are instant and nothing you enter is stored.
- Day of the year
- ISO week
- Unix time
- Time in UTC
Calculators
How the date calculators count
Date arithmetic looks simple until months of different lengths, leap years and time zones get involved. These are the rules every DateTimeHQ calculator follows, so you can check any result by hand.
How many days are in a year?
A common year has 365 days and a leap year has 366. In the Gregorian calendar a year is a leap year if it divides evenly by 4, except century years, which must also divide evenly by 400. So 2000 was a leap year, 1900 was not, and the next leap year after 2024 is 2028.
Do the calculators count the start date and the end date?
By default the calculators count from the start date up to the end date, the way most people count elapsed time, so Monday to Tuesday is 1 day. The date difference calculator has an option to include the end date as well, which adds one day and suits things like counting the days of a hotel stay or an event.
How are months counted when they have different lengths?
A month is counted from a date to the same date in the next month, such as 10 March to 10 April. When that date does not exist, the last day of the shorter month is used instead, so one month after 31 January is 28 February, or 29 February in a leap year.
Do time zones or daylight saving time change the results?
No. The date calculators work in whole calendar days, so a daylight saving change never adds or removes a day from a result. Today's date is taken from your own device, so it matches the calendar where you are.
What week number is it?
DateTimeHQ uses ISO 8601 week numbers, the international standard. ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday, so a year has either 52 or 53 weeks. The current week number is shown at the top of this page.
What is Unix time?
Unix time is the number of seconds that have passed since 00:00:00 UTC on 1 January 1970, not counting leap seconds. Computers use it to store moments in time as a single number that is the same everywhere in the world. The live value is shown at the top of this page.
Guides
Plain-language explainers with charts, checklists and quizzes, each one linked to the calculator it supports.
- Military time explained: how the 24-hour clock worksFour digits, no colon, no AM or PM. How to convert, read and say military time.
- Which generation am I? Birth year ranges from Gen Z to BoomersBirth years for every generation, why the sources disagree, and what a cusp year is.
Tools being added next
An age in weeks, months and days calculator, a next birthday countdown, dog and cat age calculators and a pregnancy due date calculator are in progress, followed by a world clock, a time zone converter, a countdown timer and an online stopwatch.
About DateTimeHQ
DateTimeHQ builds simple, accurate date and time tools. Each calculator explains the method it uses, gives a worked example you can check, and runs entirely on your device with no account and no tracking of the dates you enter. Read more about how the site works.
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