What each figure means
Enter any date and the calculator reports four things about it. The weekday is the plain day name. The day of the year counts from 1 on 1 January up to 365 or 366 on 31 December. The ISO week number follows the international standard. The leap-year flag tells you whether that year has a 29 February.
The weekday itself is derived directly from the calendar, so it is correct for any date the calculator accepts, past or future, including dates centuries away.
How ISO week numbers work
The ISO 8601 week numbering used here has two rules that differ from a casual count: weeks begin on Monday, and week 1 of a year is the week that contains that year’s first Thursday (equivalently, the week containing 4 January).
A consequence is that the first few days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next year. This is intentional and is widely used in business, manufacturing and scheduling systems, so it may not match a simple "divide the day of year by seven" estimate.
The leap-year rule
A year is a leap year when it is divisible by 4, except for century years, which must also be divisible by 400. So 2000 was a leap year but 1900 was not, and 2024 is while 2100 will not be.
This Gregorian rule keeps the calendar aligned with the solar year to within about one day every 3,000 years. The flag is handy for checking whether a given year contains 29 February before scheduling around it.
Formula
leap = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0Frequently asked questions
- Why might the ISO week number differ from a simple count?
- ISO weeks start on Monday and week 1 is the week containing the year’s first Thursday, so early-January dates can belong to the last week of the previous year.