It's a prominent concern in programming, but not only relevant there.

In my native language German, we do dd.mm.yyyy, which is the order you seem to use as well.

Other languages speak it as month, day, year. A different commenter suggested using that.

If you read left to right, is putting the date first purely because you're used to it, or is there a different reason you can give for it?

Someone mentioned "relevance", but where do you draw the line? You can only skip month and year if you know they're the current month and year. But you can only know that after reading them.

When we read 123, we read it as one hundred twenty three, not three twenty one hundred. It's the same hierarchical structure, from bigger increments and scope to smaller, higher specificity. If you care only about 10k vs 20k, you can skip the rest of the numbers.

The same goes for dates. It makes sense to go from broad to specific, in the reading direction. When reading left to right, that's YYYY-MM-DD. I'm familiar with the current year and month, and so I can categorize at a glance. For dates further away, I can read as deep as the specificity is relevant to me - in reading order.

This, for me, is the logical hierarchical argument.

As for the sorting, simple alphabetical sorting sorts YYYY-MM-DD into time order, but not DD.MM.YYYY.

This applies not just to programming, but to all forms of text or date formatting.

If you're sorting by date in DD.MM.YYYY, you're reading and parsing it right to left. If it's a date within text or pre-text, you switch the reading direction for a part of the text/title/whatever it is.

Not just programming. e.g. if you create a bunch of files or folders on your computer and they're sorted by name, then if you do d/m/y they'll all be out of order. y/m/d will be in order.

But yeah, for human contexts usually day is more important.