I'm not sure when this changed, but I just created a 24.10.0 build with the Firmware Selector and when it succeeded, the date was in American format (M/D/YYYY) instead of ISO 8601 (YYYY-MM-DD) previously.
Please change this back for the sake of sanity...
Which date and where?
When the build succeeds, it returns a little block of info that includes the device model, target, firmware version, and date (below the model selector box and above the download links). That date is the one in the wrong format. It used to be ISO 8601.
It changed to match the browser locale.
Well that's annoying. Now I have to figure out how to fine-tune Firefox's locale settings so I get ISO 8601 while still using the US English base.
Why did someone think that was a good idea?
And apparently I can't do that in FF 135 on Linux. There's an about:config preference to allow OS locales but it doesn't appear to work anymore.
So if I want to get rid of the idiotic US date format, I literally have to reinstall Firefox with the en-GB locale and live with whatever other locale settings that changes.
Not cool. 
I'm not sure you're going to get any sympathy with this attitude on a forum of a volunteer-run project. Just because you disagree with something, doesn't make it idiotic. Especially a locale used by hundreds of millions of people.
And as for the locale issue itself, as another Firefox user, perhaps it's Firefox that's to blame here for not following the OS locale?
The US date format is objectively idiotic. That's not a commentary on the OpenWrt project.
I agree that this is a Firefox issue as well. It's tremendously bad design on Mozilla's part to not just follow OS locale by default, and allow customization beyond that.
However, at the end of the day, the question is why this change was made here in the first place. What was wrong with server date & time?
You can easily change Firefox's locale on Linux using:
LC_TIME=en_CA firefox
en_CA uses ISO-8601 as the date format. You can check your current browser locale information here.
I leave the exercise of making this change permanent in your DE of choice up to you.
It looks like there is quite a bit of date diversity globally. https://en.wikipedia.org/wiki/List_of_date_formats_by_country
The ISO 8601 format YYYY-MM-DD (2025-03-14) is intended to harmonize these formats and ensure accuracy in all situations. Many countries have adopted it as their sole official date format, though even in these areas writers may adopt abbreviated formats that are no longer recommended.
Didn't work, even after I allegedly installed the en-CA locale pack. Even if it did, it's only a band-aid for a problem that shouldn't really exist to begin with. There's no good reason not to leave build date & time in ISO-8601 and GMT.
You are free to submit a PR to the firmware selector on GitHub with your reasoning and opinion.
And I tested my workaround before suggesting it: the date format was switching according to the locale.
In France, I can see DD/MM/YYYY, obviously according to local settings.
Even if displayed in US format, is it so hard to figure it out?
Any cultural topic is subjectively idiotic when trying to compare to another culture. It should be considered with the scope of the original culture, in which it makes sense. You can argue it's akward, impratical, inconvinient, but not idiotic: your quote can been seen as disrespectful.
Here are some informations on the matter.
https://iso.mit.edu/americanisms/date-format-in-the-united-states/
Ask them.
The firmware selector is meant for the end user. As such a local date format is more suitable.