What this calculator does
This calendar calculator answers the questions a plain paper calendar cannot at a glance: which day of the week a date falls on, which numbered week of the year it sits in, which quarter it belongs to, and how many days into the year it already is. Enter any date, past, present or future, and every figure updates from it.
These are the same underlying calculations that scheduling software, payroll systems and financial reporting quietly run in the background. A quarter boundary decides which reporting period a transaction lands in, and an ISO week number is how many businesses and government forms label weeks instead of dates.
The formula
The day of the week comes straight from the date itself. The quarter is the month divided by three, rounded up. Day of the year counts every day from 1 January up to and including the date entered, and days remaining is simply the length of that year minus the day of the year. The ISO week number follows the international standard: week 1 is the week containing the year's first Thursday, so the last few days of December can sometimes belong to week 1 of the following year, and the first days of January can belong to the last week of the previous year.
| Term | Meaning |
|---|---|
| Day of the year | How many days have elapsed since 1 January of that year, counting the date itself as day 1. |
| ISO week number | The week number under the ISO 8601 standard, where week 1 always contains the year's first Thursday and weeks run Monday to Sunday. |
| Quarter | One of four three-month blocks of the year: Q1 (Jan-Mar), Q2 (Apr-Jun), Q3 (Jul-Sep), Q4 (Oct-Dec). |
| Leap year | A year with 366 days instead of 365, occurring in years divisible by 4, except century years not divisible by 400. |
The inputs explained
| Field | What to enter |
|---|---|
| Date | Any calendar date. Defaults to today. |
When to use it
Working out which quarter a date falls in
Financial reporting, tax deadlines and sales targets are usually set by quarter. Entering an invoice or transaction date here shows immediately which quarter it belongs to, without counting months on your fingers.
Finding the ISO week number for a form or timesheet
Many payroll systems, shipping schedules and European date formats label periods by week number rather than by date range. This calculator converts any date straight to its ISO week.
Checking whether a year is a leap year
Century years are the trap: 2000 was a leap year, but 1900 was not, because leap years must be divisible by 400 whenever they are also divisible by 100.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Day of the week, quarter and ISO week across a year
The same date fields evaluated for five dates spread across a non-leap year.
| Date | Day of the week | Quarter | ISO week number | Day of the year |
|---|---|---|---|---|
| 2026-01-01 | Thursday | Q1 2026 | 1 | 1 of 365 |
| 2026-03-31 | Tuesday | Q1 2026 | 14 | 90 of 365 |
| 2026-07-04 | Saturday | Q3 2026 | 27 | 185 of 365 |
| 2026-12-25 | Friday | Q4 2026 | 52 | 359 of 365 |
| 2026-02-14 | Saturday | Q1 2026 | 7 | 45 of 365 |
How a leap year changes the day-of-year count
Two dates in 2024, a leap year, showing how the extra day in February shifts every later date-of-year figure.
| Date | Leap year? | Day of the year | Days remaining in the year |
|---|---|---|---|
| 2024-02-29 | Yes, 2024 has 366 days | 60 of 366 | 306 |
| 2024-12-31 | Yes, 2024 has 366 days | 366 of 366 | 0 |
Questions
What is the difference between a calendar calculator and a date calculator?
A date calculator adds or subtracts a span of time from a starting date, answering "what date is 90 days from now". This calendar calculator instead looks up facts about one date you already have, such as its weekday, week number and quarter.
Why does the ISO week number sometimes not match the calendar week I expect?
ISO 8601 weeks always run Monday to Sunday and week 1 is defined as the week containing the year's first Thursday. Because of this, the first few days of January can belong to the final week of the previous year, and the last few days of December can belong to week 1 of the next year.
How is the quarter of a date worked out?
The month number is divided by three and rounded up: January to March is Q1, April to June is Q2, July to September is Q3, and October to December is Q4, regardless of the calendar year.
Which years are leap years?
A year is a leap year if it divides evenly by 4, unless it also divides evenly by 100, in which case it is not a leap year, unless it divides evenly by 400 as well, in which case it is. That is why 2000 was a leap year but 1900 and 2100 are not.
To add or subtract a span of time from a date instead of looking one date up, use the date calculator. For the number of days between two different dates, see the days calculator.