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.