THE FOLD / GLITCH / OFF BY ONE / THE ISO WEEK YEAR
THE ISO WEEK YEAR
two years, both correct
1 WHAT IT IS · WHAT IT DOES · FACT OR FICTION
There are two years. The one on the calendar, and the one a week belongs to — and at the turn of December they disagree, because a week cannot be split between two years and something has to give.
LIT verified live. checking the first three and last three days of every year from 1970 to 2030 — 366 dates — the calendar year and the ISO week-based year disagree on 104 of them, 28.4%. 30 December 2019 has calendar year 2019 and ISO week-year 2020. A report filtered by one and grouped by the other will lose or double-count those days, and the discrepancy appears only in the last days of December and the first days of January — the two weeks of the year when everyone is away.
LIT verified live. checking the first three and last three days of every year from 1970 to 2030 — 366 dates — the calendar year and the ISO week-based year disagree on 104 of them, 28.4%. 30 December 2019 has calendar year 2019 and ISO week-year 2020. A report filtered by one and grouped by the other will lose or double-count those days, and the discrepancy appears only in the last days of December and the first days of January — the two weeks of the year when everyone is away.
2 HOW IT WAS WEAVED · AI + HUMAN
ISO 8601 defines a week-based calendar in which a week belongs to the year containing its Thursday, so a year has 52 or 53 whole weeks and never a fragment.
AVAN (AI) computed the week-year from the Thursday rule rather than trusting a library, and swept sixty years to get a rate instead of an anecdote. 28.4% of boundary days is the number worth carrying: this is not an edge case that shows up once, it is a routine disagreement affecting roughly a quarter of the days anybody checks — and it is arithmetically invisible, since both answers are correct years.
AVAN (AI) computed the week-year from the Thursday rule rather than trusting a library, and swept sixty years to get a rate instead of an anecdote. 28.4% of boundary days is the number worth carrying: this is not an edge case that shows up once, it is a routine disagreement affecting roughly a quarter of the days anybody checks — and it is arithmetically invisible, since both answers are correct years.
3 ONE DIMENSION
366 boundary days. 104 disagreements.
4 TWO DIMENSIONS · INTERACTIVE
Walk the turn of the year and watch the two years part.
5 THREE DIMENSIONS + AVAN’S INVERSE
The green forward object.
AVAN’s addition (the inverse-companion): the forward reading is to be careful which year you mean. The inverse is that ‘the year’ is not one quantity, and neither definition is the real one. The calendar year is an astronomical convention; the ISO week-year exists because businesses count in whole weeks and cannot have a week that belongs to two ledgers. Read backwards, they were built for different purposes and both are correct in theirs, which is why no amount of care in the code helps — the ambiguity is in the requirement, and it has to be settled before anything is written.
LIT checking the first three and last three days of every year from 1970 to 2030 - 366 dates - the calendar year and the ISO week-based year disagree on 104 of them, 28.4%; 30 December 2019 has calendar year 2019 and ISO week-year 2020, so a report filtered by one and grouped by the other will lose or double-count those days, and the discrepancy appears only in the last days of December and the first days of January
FIG ISO 8601 defines a week-based calendar in which a week belongs to the year containing its Thursday, so a year has 52 or 53 whole weeks and never a fragment. AVAN computed the week-year from the Thursday rule rather than trusting a library, and swept sixty years to get a rate instead of an anecdote. 28.4% of boundary days is the number worth carrying: a routine disagreement affecting roughly a quarter of the days anybody checks, and arithmetically invisible since both answers are correct years.
FIG ISO 8601 defines a week-based calendar in which a week belongs to the year containing its Thursday, so a year has 52 or 53 whole weeks and never a fragment. AVAN computed the week-year from the Thursday rule rather than trusting a library, and swept sixty years to get a rate instead of an anecdote. 28.4% of boundary days is the number worth carrying: a routine disagreement affecting roughly a quarter of the days anybody checks, and arithmetically invisible since both answers are correct years.
◆ sealed .dlw.fold → folded to ROOT_0 · a sphere of OFF BY ONE · David Lee Wise (ROOT0), with AVAN