Short answer
ISO week-year vs calendar year
Why the ISO week-year can differ from the calendar year around New Year.
Short answer
A calendar year changes on 1 January; an ISO week-year changes only at a week boundary.
How it works
Write the year and week together, such as 2026-W01, to remove ambiguity.
Three year-boundary checks
31 December 2018 is 2019-W01. 1 January 2021 is 2020-W53. 4 January 2027 is 2027-W01. In each case the Thursday identifies the ISO week-year.
Only late December and early January can differ from the calendar year. Store the week-year together with the week number, for example 2026-W53. Pairing a date's calendar year with its ISO week number can split or duplicate report groups. Grouping by the week's Monday is another unambiguous option.