January 1st is not always on the first week of the year

Obligatory links:

Falsehoods that programmers think about dates and times: https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time

Tom Scott descends into time and timezone madness: https://www.youtube.com/watch?v=-5wpm-gesOY

I've personally never worked with ISO 8601 directly, and quite frankly prefer how RFC 3339 simply doesn't deal with weeks at all.

fuck sake.

thanks for the heads up

Week numbers can also vary by country. Not always, but sometimes, as we're not all in agreement on what constitutes the first week of the year.

Something most people in international organisations and teams get to find out the hard way at least once.

If you think looking at Thursday seems like a random choice: It’s basically a majority vote among the days of the week. If Thursday is in the new year, then 4 days (Thu–Sun) are in the new year. If Thursday is in the old year, then 4 days are in the old year (Mon–Thu).

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

I hate working with date/time.

Similarly, developers have many misconceptions about addresses: https://www.mjt.me.uk/posts/falsehoods-programmers-believe-about-addresses/

When we implemented week numbers we actually had to support both systems because customers expected the number to match that in their other applications - some of which followed the standard, and some didn’t.

So nobody talks about leap seconds? 😅

Did they settle for Thursday as the "middle of the week" specifically because ISO 8601 starts the week count on Mondays?

Also: January is not always the 1st month, sometimes it is the 0th.

1/1/2026 can be both Jan 1st, and Feb 1st.

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For the downvoters, try it in your browser's terminal:

let test = new Date("1-1-2026");

console.log(`Year: ${test.getFullYear()}, Month: ${test.getMonth()}, Day: ${test.getDate()}`);
// Prints -> Year: 2026, Month: 0, Day: 1

// --- --- --- --- --- --- --- ---

// test.getFullYear() returns the year, but test.getYear() only returns the number of years since 1900

// test.getMonth() returns the month, but the first month is 0-indexed

// test.getDate() returns the day, 1-indexed, but test.getDay() returns the current day right now and not the day of the date object