Question

I saw in PHP’s documentation that there are two ways to format the year value in 4 digits:

Y – A full numeric representation of a year, 4 digits

o – ISO-8601 year number. This has the same value as Y, except that if the ISO week number (W) belongs to the previous or next year, that year is used instead. (added in PHP 5.1.0)

I researched more what the ISO-8601 is and it seems to be more "useful" or even accurate(?) based on php’s documentation. I think it’s more useful because it prevents ambiguity (like when handling date values like 1981-04-05).

But does it really make a difference? Is there some sort of "best practice"? Is ‘o’ slower that’s why people use Y instead when they’re not really doing anything complicated with the dates?

What specific use-cases perhaps is this useful?

Thank you for your help.

EDIT:
Thank you Jason for an example! I guess why I brought this up as well is I’m wondering why not just use ‘o’ all the time? I’m thinking I should just use ‘o’ all the time for currently unknown future specification that will require me to do more complex date operations.

Answer 1

ISO 8601 is a very useful standard for representing dates, but it has several components, not all of which are useful for many purposes. In particular, the “ISO week” and “ISO year” can be slightly confusing terms, and they’re often not what you really want to use (and if you do want to use them, you probably already know that).

Wikipedia has a pretty good explanation. Basically, the ISO 8601 standard defines a separate calendar that, instead of breaking up into months and days of the month, breaks up into weeks and days of the week.

You might think this wouldn’t make a difference for specifying the year, but there’s a little wrinkle: which year do the weeks near the beginning and end of the calendar year fit into? As Wikipedia says, it’s not quite intuitive: “The first week of a [ISO] year is the week that contains the first Thursday of the [calendar] year.”

The practical upshot: If you’re representing dates as conventional year-month-day values, you don’t want to use the “ISO year” (o). You won’t notice the difference for most of the year, but as you get to December and January, suddenly you’ll (sometimes) end up with dates that are a year off. If you just test your system with current dates, you can easily miss this.

Instead, you probably just want to be using plain old Y-m-d. Even though the date() documentation doesn’t specifically say it, that’s a perfectly valid ISO 8601 standard date.

Really, you should only be using o if you’re using the ISO “week date” calendar (meaning you’re likely also using W for the week number and N for the day-of-week).