Several of the functions in this plugin make use of ISO 8601 week numbers. This system is unfamiliar to many people; here’s a brief summary of how it works.
In the ISO 8601 standard, a “week” is defined as a seven-day period, always beginning on a Monday and always ending on a Sunday. (This differs from the traditional week running from Sunday through Saturday.) The fourth and central day of any ISO week is therefore a Thursday.
Because New Year’s Day does not always fall on a Monday, it’s possible for the year to change in mid-week. In other words, an ISO week can begin in one year and end in another. ISO 8601 defines a week as belonging to the year in which its Thursday falls. Think of Thursday as the “anchor” of the ISO week: the entire week is tied to the year where Thursday drops anchor.
ISO weeks are numbered starting with 1 (not zero.) Week 1 of a year is the week containing the first Thursday of the year. Week 2 is the week containing the second Thursday of the year, week 3 contains the third Thursday, and so on. The last ISO week of a year is numbered 52 or 53, since some years contain 52 Thursdays and others have 53.
As many as three days at the start of January may belong to the final week of the previous year; as many as three days at the end of December may belong to the first week of the following year. For ISO week-numbering purposes, any date falls in the same week as the nearest Thursday.
For example, consider Friday, December 31, 2010. The closest Thursday is December 30; so December 31 falls in the last ISO week of 2010. If we look at Sunday, January 2, 2011, we see that the nearest Thursday is still December 30, 2010; so January 2, 2011 is also in the final week of 2010. But for Monday, January 3, 2011, the closest Thursday is January 6 — the first Thursday of the new year — so January 3, 2011 falls in the first ISO week of 2011.
Year | Week | Mon | Tue | Wed | Thu | Fri | Sat | Sun |
2010 | 50 | Dec 13 | Dec 14 | Dec 15 | Dec 16 | Dec 17 | Dec 18 | Dec 19 |
2010 | 51 | Dec 20 | Dec 21 | Dec 22 | Dec 23 | Dec 24 | Dec 25 | Dec 26 |
2010 | 52 | Dec 27 | Dec 28 | Dec 29 | Dec 30 | Dec 31 | Jan 1 | Jan 2 |
2011 | 01 | Jan 3 | Jan 4 | Jan 5 | Jan 6 | Jan 7 | Jan 8 | Jan 9 |
2011 | 02 | Jan 10 | Jan 11 | Jan 12 | Jan 13 | Jan 14 | Jan 15 | Jan 16 |
You can use QCAL /I
to display ISO 8601
week numbers.