Calendar Calculator

This tells you everything about one date rather than measuring a gap between two. Which day of the week it falls on, which day of the year it is out of 365 or 366, which ISO week it belongs to, how many days remain in the year and what proportion of the year has passed. Each is a small fact and together they answer most of what people want from a date lookup. Day of the week is the common one, whether for a date in the past you are trying to place or a future one you are planning around. Day of the year matters for anything accrued daily, since leave, interest and depreciation all care which of 365 days you are standing on. The proportion of the year elapsed is useful for reading a year-to-date figure honestly: revenue that is 40 percent of target in a year that is 55 percent gone is a different story from the same number in March. The date is read as a plain calendar date with no time zone applied, so the answer is the same wherever it is opened, and leap years are handled under the full Gregorian rule including the century exception that catches out manual calculations.

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Monday
day of the week
Day of the year166 of 365
ISO week number25
Days remaining in the year199
Year elapsed45.5%
Leap yearNo

Read as a plain calendar date with no time zone applied, so the result is the same wherever the page is opened. Leap years follow the full Gregorian rule.

How it works

The day of the week comes from the date itself. The day of the year is the number of days from 1 January to the date, plus one so that 1 January is day 1. Days remaining subtracts that from the year length, which is 365 or 366. The proportion elapsed is the day of the year divided by the year length. The ISO week number is found by taking the Thursday of the week the date falls in and counting weeks from the first Thursday of that year, which is what makes the numbering work correctly across a year boundary.

Worked example

Take 15 June 2026, the default. It falls on a Monday. Counting from 1 January: January has 31 days, February 28 in this non-leap year, March 31, April 30 and May 31, which totals 151, plus 15 days into June gives day 166. The year has 365 days, so 199 remain and 45.5 percent has elapsed. It falls in ISO week 25. Had 2026 been a leap year, every date after February would shift by one day of the year.

Related calculators

Data sources: week numbering follows ISO 8601. Leap years follow the Gregorian rule: divisible by four, except century years unless divisible by 400.

Frequently asked questions

What day of the week was a given date?

Enter it above and the day is shown. The calculation comes from the calendar itself rather than a formula such as Zeller's congruence, so it is correct for any date the browser can represent, including well before 1900.

What is the day of the year?

The count of days from 1 January, where 1 January is day 1 and 31 December is day 365, or 366 in a leap year. It matters for anything accrued daily, such as leave, interest or depreciation.

How is the ISO week number worked out?

Week 1 is the week containing the first Thursday of January, and weeks run Monday to Sunday. That means early January can belong to the final week of the previous year, which is why counting weeks from 1 January gives the wrong answer.