ISO-8601 Week Definition: Is January 1st, 2023 the Last Week of 2022?

Posted on by Kirin Choi

Is January 1st, 2023 (Sunday) the 1st week of 2023 or the last week of 2022?

At first glance, this looks like a very simple question but actually it is not. Because “a week is a period of 7 days” is the only definition we have about a week. Does a week start on Sunday? Or on Monday? When is the 1st week of a year, or week number 1 of a year?

How did I live without knowing the answer on a simple question like this? But if you take a breath and think about this question again, we can find there’s almost no chance to ask what is the week number of this week in daily living. Then the concept of the week number should have been faded out long long ago. But it DOES still exist and that means somewhere in this world still uses the week number actively.

Not a small amount of businesses in this world runs on weekly basis. Broadcast stations still work with weekly broadcast schedules, naturally commercials on programs are being booked weekly. Lots of retailers and wholesalers make and execute their sales plan every week. Some companies utilize weekly or bi-weekly paydays.

Above facts do not lead to the conclusion that the concept of the week number is necessary. But without the week number there would be lots of uncomfortableness especially from the businesses running on weekly basis. Without the week numbers, we have to say the week starts on Jan 16th and end on Jan 22nd instead of saying simply the 3rd week of 2023. Moreover, if we don’t have a solid definition of a week, some can insist that the 3rd week starts on Jan 15th not on Jan 16th. Therefore to avoid unnecessary conflicts and chaos in communications a solid definition of a week and a week number is definitely necessary.

Actually there IS a standard for this, ISO-8601. It is a standard for date and time data exchange, which covers the definitions of year, month and day, and its notation. And it does include the definition of a week and a week number. Per this standard;

  • A week starts on Monday and ends on Sunday
  • The week with the first Thursday of the year in it is the 1st of the year

According to this definition, Jan 1st, 2023 is the 52nd week or the last week of the year of 2022.

ISO standard is not the only way to define a week number. For example. there is something called a Broadcast Calendar. This was a calendar widely used from American broadcast stations. It defines Monday as the 1st day of a week and the week having Jan 1st in it as the 1st week of a year. According to this definition, Jan 1st, 2023 is the 1st week of the year of 2023. And Dec 31st, 2022 is also the 1st week of the year of 2023.

There is also 4-5-4 Calendar which defined by National Retail Federation, NRF and used by various retailers and wholesalers. A week starts on Sunday, the 1st week of a year starts in Feb. A year is consist of 52 weeks, but if the next week of the 52nd week have 4 or more days of Jan, it is defined as the 53rd week. This calendar helps retailers and wholesales to compare year-to-year sales and plan. Per this calendar, Jan 1st, 2023 is the 49th week of the year of 2022.

There are many more definitions on the week numbers but we don’t have to list all of them here. And there could be no calendar that perfectly matches my needs. In that case, I’d better have a new definition of my own.

References

Leave a comment