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.
更新于 已验证的示例：5
Tue 29 Sep 2026 is in ISO week 40 of 2026 (2026-W40-2), day 272 of 365 with 93 days left, in Q3.
Tue 29 Sep 2026 is a Tuesday, ISO weekday 2 (Monday = 1).
ISO weeks run Monday to Sunday and belong to the year that contains their Thursday. This week's Thursday is Thu 1 Oct 2026, so the week-numbering year is 2026.
274 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.
核验来源：Python 3.8: date(2026,1,1).isocalendar() = (2026, 1, 4); tm_yday = 1
核验来源：Python 3.8: date(2027,1,1).isocalendar() = (2026, 53, 5)
核验来源：Python 3.8: date(2024,12,30).isocalendar() = (2025, 1, 1); tm_yday = 365 in a 366-day year
核验来源：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.
准确性取决于输入值和方法的假设。十进制运算使用50位有效数字，但估算、数值方法和源数据的精度可能较低；显示时的舍入并不能消除这些限制。 已按独立来源核验的计算示例：5。 例如，“1 January 2026 (a Thursday)”根据Python 3.8: date(2026,1,1).isocalendar() = (2026, 1, 4); tm_yday = 1进行核验。
ISO 8601-1:2019, §5.2.4 — week dates; Microsoft Excel ISOWEEKNUM and WEEKNUM functions.
此计算器包含 5 个已解示例，答案来自独立来源。这些示例会在测试套件中运行，你也可以在此运行验证。
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.
计算两个日期之间的天数，也可按年、月、日、周和小时表示；还可排除周末及自定义假日，计算工作日。
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.