ISO 8601 Week Numbering:

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.

YearWeekMonTueWedThuFriSatSun
201050Dec 13Dec 14Dec 15Dec 16Dec 17Dec 18Dec 19
201051Dec 20Dec 21Dec 22Dec 23Dec 24Dec 25Dec 26
201052Dec 27Dec 28Dec 29Dec 30Dec 31Jan 1Jan 2
201101Jan 3Jan 4Jan 5Jan 6Jan 7Jan 8Jan 9
201102Jan 10Jan 11Jan 12Jan 13Jan 14Jan 15Jan 16

You can use QCAL /I to display ISO 8601 week numbers.