ISO 8601 should however be used to replace notations such as “2/4/95” and “9.30 p.m.”.Apart from the recommended primary standard notation YYYY-MM-DD, ISO 8601 also specifies a number of alternative formats for use in applications with special requirements.

It is not intended as a replacement for language-dependent worded date notations such as “24.

Dezember 2001” (German) or “February 4, 1995” (US English).

In addition, these formats have several important advantages for computer usage compared to other traditional date and time notations.

The time notation described here is already the de-facto standard in almost all countries and the date notation is becoming increasingly popular.

Possible ISO week numbers are in the range 01 to 53. (There is one historic exception: the year in which the Gregorian calendar was introduced had less than 365 days and less than 52 weeks.) Proof: Per definition, the first week of a year is W01 and consequently days before week W01 belong to the previous year and so there is no week with lower numbers.

Considering the highest possible week number, the worst case is a leap year like 1976 that starts with a Thursday, because this keeps the highest possible number of days of W01 in the previous year, i.e. In this case, the Sunday of W52 of the worst case year is day number 4 51*7=361 and 361-366=5 days of W53 belong still to this year, which guarantees that in the worst case year day 4 (Thursday) of W53 is not yet in the next year, so a week number 53 is possible.

In this case, the Sunday of week 52 would be day number 3 52*7=367, i.e.

only the last 367-365=2 days of the W52 reach into the next year and consequently, even a worst case year like 1999 has a week W52 including the days 1999-12-27 to 2000-01-02. Both day and year are useful units of structuring time, because the position of the sun on the sky, which influences our lives, is described by them.

Especially authors of Web pages and software engineers who design user interfaces, file formats, and communication protocols should be familiar with ISO 8601.