What ISO week is January 1, 2027 in?
January 1, 2027 is a Friday, so it falls in the LAST week of the previous ISO year, not the first week of 2027.
Worked from a fixed input so the numbers stay put: 2027-01-01. Try your own on the home page.
2027-01-01 (Friday) is in ISO week 53 of 2026.
- Calendar date
- 2027-01-01
- ISO year
- 2026
- ISO week
- 53
- ISO weekday
- 5 (Friday)
- Weeks in that ISO year
- 53
This date's ISO week-numbering year (2026) is not its calendar year (2027)
2027-01-01 falls in ISO week 53 of 2026, not 2027, because ISO week 1 is defined by which week contains the year's first Thursday, not by January 1st. This is the single most common source of disagreement between "week number" tools.
Fix: Report the ISO year alongside the week number — "week 52 of 2025", not just "week 52".
Other examples
Not a general calendar or scheduling tool, and it does not know about business fiscal-year or retail 4-4-5 week numbering, which are different systems that also call themselves "week numbers." It has no concept of time of day or timezone — it operates on calendar dates only. Not for US-style "week of the month" or non-ISO week-start conventions (e.g. Sunday-start week numbering used by some US calendars).