ISO週日曆

每个月的星期

ISO的标准并未定义任何周与月相关联的协定。月中的每一天和月，也都以周和周日表达，且不会混淆不清。
在会计年度中，周年统计受益于星期是一个显著实体的规则。因此，在实务中通常每一季固定有13个星期，并且分割成5+4+4周、4+5+4周、或4+4+5周。在53个星期的长年中，最后一季有14个星期。
虽然ISO 8601认为没有这样的需要，但在有必要将星期分配到一个月中的时候，可以应用的规则是从每年的第一个星期开始。由这种模式产生的结果，会是不规则的。有4个月（或在长年有5个月）会有5个星期，但是至少有29天是从星期四开始，30天是从星期三开始，31天从星期二开始。

缺点

The year number of the ISO week very often differs from the Gregorian year number for dates close to 1 January. For example, 29 December 2014 is ISO 2015-W1-1, i.e., it is in year 2015 instead of 2014. A programming bug confusing these two year numbers is probably the cause of some Android users of Twitter unable to login around midnight of 29 December 2014 UTC. 
Solar astronomic phenomena, such as equinox and solstice, vary over a range of at least seven days. This is because each equinox and solstice may occur any day of the week and hence on at least seven different ISO week dates. For example, there are spring equinoxes on 2004-W12-7 and 2010-W11-7.
The ISO week calendar relies on the Gregorian calendar, which it augments, to define the new year day (Monday of week 01). As a result, leap weeks are spread across the 400-year cycle in a complex, seemingly random pattern. There is no simple algorithm to determine whether a year has 53 weeks without tabular lookup. Most calendar reform proposals using leap week calendars are simpler in this regard, although they may choose a different leap cycle.
Not all parts of the world have a work week that begins with Monday. For example, in some Muslim countries, the work week may begin on Saturday, while in Israel it may begin on Sunday. In the US the work week is often defined to start on Monday, although the week itself is usually considered to start on Sunday.

其他的周数系统

For an overview of week numbering systems see week number. 
The US system has weeks from Sunday through Saturday, and partial weeks at the beginning and the end of the year, i.e. always 53 weeks. An advantage is that no separate year numbering like the ISO year is needed. Correspondence of lexicographical order and chronological order is preserved (just like with the ISO year-week-weekday numbering), but partial weeks make some computations of weekly statistics or payments inaccurate at end of December or beginning of January.
A variant of this US scheme groups the possible 1 to 6 days of December remaining in the last week of the Gregorian year within week 1 in January of the next Gregorian year, to make it a full week, bringing a system with accounting years having also 52 or 53 weeks and only the last 6 days of December may be counted as part of another year than the Gregorian year.
The US broadcast calendar counts the week containing 1 January as the first of the year, but otherwise works like ISO week numbering without partial weeks.