ISO is the International Organization for Standardization. ISO has not sanctioned an alternative to the Gregorian Calendar currently used by most of the world. But it has provided standard notations for recording Gregorian dates, one of which dispenses with months. The ISO dating notations can be found in the technical standard referred to as ISO 8601.

Dating by weeks is common in European countries; and it is becoming increasingly more common world-wide. All 52 or 53 weeks of the year receive a number, with days of the week numbered 1 through 7, beginning with Monday.

So a typical week date notation would look like this: 1995-W03-6. That notation indicates the sixth day of the third week of 1995. The "W" in the notation is optional. The "0" should precede week numbers less than 10, so all week numbers have two digits.

The difficulty in dating by weeks is fixing the number of the week at year ends and beginnings. Because years often begin in the middle of the week, a standard is needed to identify the first week of the year.

The ISO 8601 rule is: The first week of the year is the week containing the first Thursday.

So if January 1 falls on a Friday, it belongs to the last week of the previous year. If December 31 falls on a Wednesday, it belongs to week 01 of the following year.

Here is an ISO Week Date conversion algorithm.

The program initially displays the current ISO Week Date and tables showing the position of the current date in the current year. "Current" is defined by the date settings on the machine loading the program.

The tables display the calendar year in arbitrarily chosen 9-week sections.

The program will also convert Gregorian dates entered in Day-Month-Year format to the ISO Week Date format.

There are only a few restrictions on the range of convertible dates:

• Dates entered must be valid Gregorian dates no earlier than year -10000 (negative ten thousand);
• Day numbers may begin with "0" only for days 1-9 of the month;
• Year numbers may begin with "0" only for years 0-9;

The program is based on the so-called Proleptic Greogrian Calendar. This is the present-day Gregorian Calendar extended indefinitely into the past and future. So the program makes no attempt at historical accuracy prior to the Gregorian Calendar Reforms in September 1752 (Britain) or October 1582. For example, although the pre-Gregorian year 1500 was a leap year, the program treats it as a common year, in accordance with the Gregorian leap year rule.

There is some debate over the propriety of a year 0. The program will convert Greogrian dates in year 0, because its basis is a proleptic calendar. It will also convert dates prior to year 0 specified in negative numbers. So year 0 corresponds to year 1BC in history; year -1 corresponds to year 2BC in history.

The program is written in JavaScript; © 1998 by Rick McCarty. mccartyr@mail.ecu.edu