130
January 1st is not always on the first week of the year (www.jpvalencia.com)
submitted 4 months ago by to c/programming@programming.dev
43 comments fedilink hide all child comments
 

The other day at work I stumbled upon this bug and thought it was worth to write a blog post about. Spoiler: It has nothing to do with timezones!

TLDR: According to ISO standard 8601 (which is what Python's date.isocalendar().week uses for example), the first week of the year is the week with the first Thursday of the year. So sometimes the first few days of January belong to the last week of previous year, and sometimes the last few days of December belong to the first week of next year :D

you are viewing a single comment's thread
view the rest of the comments
[–] 16 points 4 months ago (2 children)

Even better: two weeks with the same week number can exist in the same year.

  • permalink
  • fedilink
  • source
  • hideshow 4 child comments
    •
  • [–] 8 points 4 months ago (1 child)

    Fuck, I just wrote an app to help HR process data and I had to, without using Ai, define the first week of the year to match that ISO spec. Week 53 was a fun one but I now have to cover two weeks with the same number.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 12 points 4 months ago (2 children)

    Yep, but the year of week will be different for each.

    Use a date library. Life is too short to make your own. The new Temporal library in JavaScript is quite good.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 4 child comments
    •
  • [–] 3 points 4 months ago (1 child)

    The new Temporal library in JavaScript is quite good.

    Sadly, Safari fucks us yet again.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 5 points 4 months ago (1 child)

    How is that? Do you mean that week 1 can occur twice? Or is there another issue?

    IIUC you can have days at the start and the end of the year that are all in week 1, and those are two different weeks. But one of those would be, let's say, week 1 year 2025, and the other would be week 1 year 2026. According to the blog post if you have a week 1 at the end of a year the ISO year number for that week is the next year.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 15 points 4 months ago (1 child)

    It only happens with ISO weeks. An example is 2006. Each of the weeks belong to different years (so 2005W52 and 2006W52 are different), but each can contain days from another year. So for example Sunday 2006-01-01 is part of week 52 of 2005. Week 1 starts on January 2. Then at the end of 2006 you have another week 52, but that week is actually part of 2006.

    It's a bit of a cheeky thing to point out, because at no point is a day in two different weeks, and the week itself only belongs to one year. It's just that you can't assume that any given day belongs to the same year as the week it is in. That is: 2006-01-01 is in 2005W52 not 2006W01.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •