Week number for any date: ISO 8601 week and week-numbering year, US week number, day of the year, days left in the year and the quarter.
Sun 27 Sep 2026 is in ISO week 39 of 2026 (2026-W39-7), day 270 of 365 with 95 days left, in Q3.
Sun 27 Sep 2026 is a Sunday, ISO weekday 7 (Monday = 1).
ISO weeks run Monday to Sunday and belong to the year that contains their Thursday. This week's Thursday is Thu 24 Sep 2026, so the week-numbering year is 2026.
267 is the Thursday's day of the year.
4 is 1 January's weekday counted from Sunday = 0; week 1 is the week containing 1 January.
ISO 8601 weeks run from Monday to Sunday, and each week belongs to the year that contains its Thursday. Week 1 is therefore the week with the year's first Thursday, which is also the week containing 4 January. The calculator finds the Thursday of the date's week, takes its day of the year d, and computes ⌊(d − 1)/7⌋ + 1. It also gives the US-style number from Excel's WEEKNUM(date, 1), where weeks start on Sunday and week 1 is the week containing 1 January.
Week numbers appear in payroll periods, production plans and European business calendars. Near New Year the two systems part: 1 January 2027 is a Friday, so it falls in ISO week 53 of 2026 (2026-W53-5), while 30 December 2024 is already ISO week 1 of 2025 but US week 53 of 2024.
Day of the year counts 1 January as day 1, so 31 December is day 365, or 366 in a leap year.
Checked against: Python 3.8: date(2026,1,1).isocalendar() = (2026, 1, 4); tm_yday = 1
Checked against: Python 3.8: date(2027,1,1).isocalendar() = (2026, 53, 5)
Checked against: Python 3.8: date(2024,12,30).isocalendar() = (2025, 1, 1); tm_yday = 365 in a 366-day year
Checked against: Python 3.8: date(2024,12,31).timetuple().tm_yday = 366
52 or 53 ISO weeks. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday; that happens in 71 of every 400 years. 2020 and 2026 have 53 weeks, and the next are 2032 and 2037. 52 weeks cover only 364 days, so the spare day or two each year build up until a 53-week year absorbs them, usually 5 or 6 years apart.
They use different rules for the first week. ISO 8601 weeks start on Monday and week 1 contains the first Thursday of the year; Excel's ISOWEEKNUM follows this rule. The US convention, Excel's WEEKNUM with return type 1, starts weeks on Sunday and makes week 1 the week that contains 1 January, even if that week has only one day of the new year.
Yes, under ISO 8601 when 1 January is a Friday, Saturday or Sunday. 1 January 2027 is a Friday, so it belongs to week 53 of 2026 and ISO week 1 of 2027 starts on Monday 4 January. 1 January 2028 is a Saturday and falls in week 52 of 2027. In the other direction, 29 to 31 December can belong to week 1 of the next year.
Only in informal use. In manufacturing and logistics a day-of-year code is often called a Julian date: 26 September 2026 is day 269, written 26269 in YYDDD form. Astronomers mean something else by Julian Day, a continuous count of days from 1 January 4713 BC in the Julian calendar; noon UTC on 26 September 2026 is Julian Day 2,461,310.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “1 January 2026 (a Thursday)” is checked against Python 3.8: date(2026,1,1).isocalendar() = (2026, 1, 4); tm_yday = 1.
ISO 8601-1:2019, §5.2.4 — week dates; Microsoft Excel ISOWEEKNUM and WEEKNUM functions.
5 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
Day of the week for any date, past or future: the weekday in the Gregorian calendar, worked through step by step with Zeller's congruence.
Days between two dates, also as years, months and days, weeks and hours, plus business days after weekends and your own holidays.
Convert a Gregorian date to Hijri (Umm al-Qura and tabular), Hebrew, Persian, Indian national (Saka) and Julian dates, or a Julian date back.
Leap year checker: whether a year has 366 days under the Gregorian rule, which test decides it, and the previous and next leap years.
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