Date, time, and schedules
Why can January belong to the previous ISO week-year?
Tools to use
A New Year report labelled 2020-W53 can look stale. For January 1, 2021, however, it is the correct ISO week identifier. Calendar year and week-based year can differ near New Year. Inspect the week's start and end instead of comparing year labels alone, so one week does not get split between two reports.
Enter January 1 and inspect both ends of its week
2021-01-01Select date-to-week mode. The week-based year is 2020, week 53, starting December 28, 2020 and ending January 3, 2021. The input date has not changed; that calendar date belongs to the seven-day interval labelled 2020-W53.
Next try 2021-01-04: it starts week 1 of 2021. Test nearby Mondays as well as January 1. Naming the December 28–January 3 report 2021-W53 combines the wrong year with its week number. Treat the date, week-based year and week number as a checked set.
Week 1 is not always the week containing January 1
ISO weeks run Monday through Sunday. Week 1 contains the year's first Thursday, equivalently January 4. This keeps weeks as seven-day intervals instead of creating a short fragment at New Year. A week whose Thursday is in the previous year can include January 1 on Friday.
The reverse also happens: December 31, 2018 belongs to week 1 of 2019. A special patch only for January dates assigned to the previous year misses December dates assigned forward. Use the returned week-based year and number together rather than attaching a week number to the calendar year.
Not every year has week 53
2020-W53Switch to week-to-range mode and enter 2020-W53. It returns December 28 through January 3. But 2021-W53 is invalid. Do not increment a week counter or copy last year's layout without checking whether that week exists; an invalid week should prompt investigation, not silent rollover.
For imported week labels, compare the returned start and end with the sender's intended interval. A Sunday-based business week can use the same number for different dates. This tool answers the ISO question, not an unstated accounting calendar or reporting cutoff.
Resolve the local date before grouping timestamps
2021-01-03T15:30:00Z
2021-01-04T00:30:00+09:00These represent the same instant: Sunday in UTC and Monday in Japan. Grouping by UTC date gives 2020-W53; grouping by the Japanese date gives 2021-W01. ISO Week accepts a date, so choose the reporting time zone first, convert the timestamp, then determine its week.
An end date of Sunday is not an instruction to stop at Sunday 00:00, which would omit most of that day. Date ranges and timestamp predicates differ. Check whether local week-start inclusive to next week-start exclusive matches your reporting rule. The calculator does not run the query or validate record counts.
Check reports with a set of New Year boundaries
2018-12-31 → 2019-W01
2020-12-28 → 2020-W53
2021-01-01 → 2020-W53
2021-01-03 → 2020-W53
2021-01-04 → 2021-W01Do not validate a fix with just one ordinary date. Check year-end, New Year's Day, Sunday and the following Monday. Convert the week back to a range and confirm the original date lies inside it. This catches cases where the number is right but the boundary is shifted.
When sharing, include the dates alongside the identifier, such as ISO 2020-W53 (December 28–January 3). Readers need not know the convention to check the interval. The goal is not a familiar-looking year label but confirmation that two reports cover the same seven days.