Standard Week Number

The International Standards Organisation have a rather unusual definition for working out week numbers.

Not only do the week numbers that the ISO has defined assume the week runs from Monday to Sunday when the days of the week are actually named from Saturday to Friday (and JavaScript numbers them from Sunday to Saturday) but where there are fewer than four days from the current year in their week then those days are adssumed to belong to a week in a differet year. This makes calculating the ISO standard week number slightly more complicated unless you add appropriate date methods to your date objects.

Standard Week Number