O tempora! O mores! [Oh what times! Oh what standards!]
—Cicero: In Catilinam (63 B.C.E.)
The International Organization for Standardization (ISO) calendar, popular in Sweden and other European countries, specifies a date by giving the ordinal day in the week and the “calendar week” in a Gregorian year. The ISO standard [1, sec. 2.2.10] defines the calendar week number1 as the
ordinal number which identifies a calendar week within its calendar year according to the rule that ...