Date & Calendar Calculators

How do date calculations work?

Most date calculations reduce to counting days on the Gregorian calendar; the only subtleties are whether the first and last days are counted, how months of different lengths roll over, and which days count as working days.

Counting the days between two dates

A standard day count subtracts the earlier date from the later one, so the start date is counted and the end date is not. From January 1 to December 31, 2026 is therefore 364 days. If you want to include both the first and the last day, as you would when counting the days of an event, add one. The days between dates calculator does this for any two dates.

Adding days, months and years

Adding days is exact: the count simply rolls over month and year ends, so September 25, 2026 plus 100 days is January 3, 2027. Adding months is less precise because months have 28 to 31 days, and a date such as January 31 plus one month has no exact answer. Most systems move it to the last day of the shorter month. The add days to date calculator handles the rollover for you.

Age and business days

Age is the number of complete years since the date of birth, so it goes up only on or after the birthday. Someone born on January 1, 2000 turned 26 on January 1, 2026, which the age calculator confirms. Business days exclude Saturdays and Sundays: September 2026 has 30 days and 22 weekdays. Public holidays differ by country and region, so subtract any that apply from the weekday count.

How do you calculate someone’s exact age?

Count the complete years since the date of birth, then the complete months since the last birthday, then the days left over. Age only goes up on or after the birthday itself, so the year count is the current year minus the birth year, minus one if this year’s birthday has not arrived yet.

Worked example: someone born on March 15, 1990 is 36 years, 6 months and 11 days old on September 26, 2026. Their last birthday was March 15, 2026; six complete months later brings you to September 15, and 11 more days reach September 26. Counted another way, they have been alive for 13,344 days, which is 1,906 weeks and 2 days. The age calculator does this for any birth date, and the days between dates calculator gives the total-days view.

Edge cases

• Leap-day birthdays: someone born on February 29 has a calendar birthday only in leap years. Software and legal systems differ on whether the new age begins on February 28 or March 1 in common years, so check the rule that applies to you.
• Month ends: a month is not a fixed number of days, so “1 month and 3 days” can span 31 to 34 days depending on where it starts. Total days is the unambiguous measure.
• Different age systems: some traditional East Asian systems count a newborn as one year old. South Korea standardized on international age for legal and administrative purposes in 2023.

How do leap years and ISO week numbers work?

A leap year adds February 29 in every year divisible by 4, except century years not divisible by 400, and ISO 8601 numbers weeks from Monday, with week 1 being the week that contains the year’s first Thursday.

The Gregorian calendar, introduced in 1582 and now the civil calendar almost everywhere, has 365 days in a common year and 366 in a leap year. The extra day, February 29, keeps the calendar aligned with the solar year.

The leap-year rule

1. A year divisible by 4 is a leap year.
2. Except a year divisible by 100, which is not a leap year.
3. Except a year divisible by 400, which is a leap year after all.

So 2024 and 2028 are leap years, 1900 and 2100 are not, and 2000 was. The rule produces 97 leap years every 400 years, giving an average year of 365.2425 days and an average month of 30.436875 days. That average is what the weeks to months converter uses: 12 weeks is 84 days, or about 2.76 months.

How ISO 8601 numbers the weeks

Under ISO 8601, weeks start on Monday and week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4. A year therefore has 52 or 53 weeks, and the first or last few days of January or December can belong to a neighboring year's week. 2026 began on a Thursday, so it has 53 ISO weeks: December 28, 2026 to January 3, 2027 is week 2026-W53.

ISO dates can be written as year, week and weekday, so Friday, September 25, 2026 is 2026-W39-5. Many calendar apps can display ISO week numbers, which makes it easy to check the week of any date. Some US calendars start weeks on Sunday and count the week containing January 1 as week 1, so their numbers can differ near the start and end of a year.

How long is a year in days, weeks, hours and seconds?

A common year has 365 days, which is 52 weeks and 1 day or 8,760 hours, and a leap year has 366 days, which is 52 weeks and 2 days or 8,784 hours. Because the Gregorian calendar has 97 leap years in every 400, the average year is 365.2425 days and the average month is 30.436875 days, the figures used when a calculator converts weeks or days into months and years.

The 400-year cycle is exactly 20,871 weeks long, so the calendar repeats itself perfectly every four centuries: January 1, 2000 was a Saturday, and so will be January 1, 2400. For converting smaller spans, such as seconds to minutes or hours to days, use the time converter.

What are Unix timestamps and Julian dates?

A Unix timestamp counts the seconds since 00:00:00 UTC on January 1, 1970, while a Julian Day counts days and fractions of a day since noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar.

Computers rarely store dates as day, month and year. Instead they count time from a fixed starting point, called an epoch, which makes date math a matter of simple subtraction.

Unix time

A Unix timestamp is the number of seconds since the Unix epoch, 00:00:00 UTC on January 1, 1970, not counting leap seconds. Midnight UTC on January 1, 2026 is 1,767,225,600, and each day adds exactly 86,400. JavaScript and many APIs use milliseconds instead, so a 13-digit value should be divided by 1,000 first. To turn a timestamp into a date by hand, divide it by 86,400 to get whole days since the epoch, then add them to January 1, 1970 with the add days to date calculator.

Older systems that store Unix time as a signed 32-bit integer can count only up to 2,147,483,647, which is reached at 03:14:07 UTC on January 19, 2038. This is known as the year 2038 problem, and modern systems avoid it by using 64-bit time.

Julian Day Numbers

Astronomers use the Julian Day, a continuous count of days that starts at noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar. Each Julian day begins at noon, so noon UT on January 1, 2026 is JD 2461042, and the Unix epoch is JD 2440587.5. To convert, divide a Unix timestamp by 86,400 and add 2440587.5.

Unix time and Julian Days are both kept in UTC or Universal Time, so the local date can differ by one day; the time zone converter turns UTC into local time.