Time Utilities

ISO Week Number

Calculate the ISO week, weekday, day of year, and quarter for a date.

ISO week number and date details
ISO 8601 · Weeks start on Monday
Calculated details
2026-08-24 → 2026-08-30
2026-W35ISO week
SundayWeekday
242Day of year
Q3Quarter
Week starts
2026-08-24
Week ends
2026-08-30
The calendar year containing this date is not a leap year.

ISO Week Number

Calculate ISO 8601 week information. Weeks start on Monday, and week 1 contains the year's first Thursday, so dates near a year boundary can belong to the adjacent ISO week-year.

Output

  • ISO week-year and week number
  • Monday and Sunday boundaries
  • Weekday, day of year, quarter, and leap-year state

Frequently Asked Questions

Why does the same date have different week numbers in different systems?
Three competing definitions: ISO 8601 (weeks start Monday; week 1 contains the first Thursday), the North American convention (weeks start Sunday; week 1 contains January 1), and plain 7-day counting. Project management and European business use ISO; US media often use the American style — the tool defaults to ISO with a switch.
What is the ISO week-year boundary trap?
December 29–31 can belong to week 1 of the next ISO year, and January 1–3 to week 52/53 of the previous one. When coding, key schedules by "ISO week-year + week", never calendar year + week, or cross-year planning slips by a week.
Can a year really have 53 weeks?
Yes — an ISO year has 53 weeks when January 1 is a Thursday, or when a leap year starts on Wednesday (2020 and 2026 are examples). Weekly reports and planning templates must handle 53-week years or the end of the year goes missing.