Handwritten, I often use the military style 09 Oct 2023 just because the meaning is inarguable, but for files and such, it's always YYYY-MM-DD.
I'd also like to see us adopt GMT, just calling noon and midnight whatever number they happen to be in our location. With remote work and jet travel, it makes sense.
And base-12 arithmetic, but that's just a pipe dream.
I'm not quite sure what that means - or that we should lead with that bit, but an advantage is an advantage... Even if it only benefits Japanese readers.