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The ISO document introduces several concepts that require some explanation. Two in particular are important: |
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1.) Period. ISO distinguishes a point in time and a duration, which is the length of time between a beginning point and an ending point. On this basis, ISO introduces a "period" as consisting of a beginning point, an ending point, and the duration between the two points. A typical ISO format explicitly declares only two of these three elements, leaving the third as implied, since it may be derived from the other two. |
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2.) Week. ISO makes heavy use of the week as a defined unit, departing from U. S. practice in several ways: |
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2.1.) The calendar week starts on Monday, not Sunday. |
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2.2.) The first calendar week of a year is the week that includes the first Thursday of the year. As a consequence, the first week of a year can include days in the previous year, and the last week of a year can include days in the following calendar year. |
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2.3.) Calendar weeks are numbered starting with 1. |
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2.4.) The rigid definition of a week as a calendric unit allows dates to be identified using a yearweekday format, in addition to the more familiar yearmonthday scheme. |
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There are several problems in the ISO treatment of time which bear some examination: |
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Major time units are frequently defined in two or more ways which may seem mutually contradictory. |
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Section 3.1.2 of the ISO 8601 document defines a calendar month as one of the 12 usual Gregorian months, but then adds the note that, "In certain applications, a month is regarded as a period of 30 days." |
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A similar problem exists for the ISO year. Sections 3.18 through 3.21 define a calendar year as 365 or 366 days. Section 5.5, however, defines a period of time in a way that requires that durations employ standard units, in which a month equals 30 days and a year equals 12 months. The explicit definitions of these units are found in section 5.5.3.2.1 The consequence of these definitions is a year of 360 days. |
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In general, the definition of calendar month and calendar year do not match the definitions of month and year used as units for the calculation of durations. |
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All of this can be sorted out, if one understands that ISO usage mixes units that have the same name but not the same definition. A month, when used as a counting unit for duration calculations, has a uniform length. A month, when used as a calendric indicator unit for identifying point time, has a variable length. When the two essentially different units are mixed in a period, the result may be confusing. |
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