How it works
The day of the week is read directly from the date you enter. The ISO week number follows the international standard: weeks run Monday to Sunday, and week 1 of a year is the week containing that year's first Thursday.
day of week = the calendar's native lookup for the date entered
ISO week number = the week containing the date's nearest Thursday, counted from week 1 of that Thursday's year
Worked example
25 December 2026.
- 25 December 2026 falls on a Friday.
- It's day 359 of the year.
- Its nearest Thursday is 24 December, placing it in ISO week 52.
25 December 2026 is a Friday — day 359 of the year, ISO week 52.
Common questions
Why does the ISO week number sometimes belong to a different year than the date's own year?
The first few days of January can fall in the last ISO week of the previous year, and the last few days of December can fall in week 1 of the next year — this is a known property of the ISO 8601 standard, not an error. It happens because week 1 is defined by whichever year contains that week's Thursday.
Does this work for dates far in the past or future?
Yes — it works for any date within the range the browser's native date object supports, which covers many thousands of years in both directions, far beyond any practical use.