According to MDN, the displayed format of input type=date can change according to the browser's locale. My guess is that mail clients might do something similar when interpreting HTML. Not sure if this can be changed (without some JS running on client side). Anyway, when submitted, the value is always sent in ISO 8601 format (yyyy-mm-dd, regardless of the displayed format), which can be properly handled at the backend

Hello @hkotsubo, I don't understand why this ISO standard developers wanted to started from the end :) Also, sadly I almost don't have control about the backend of the email client and use it pretty much "out of box".

Starting from the end means dates that are treated as strings sort correctly. Ever find yourself with a directory full of files named (or prefixed/suffixed) with things like "mar-5-2019" or "12-apr-2020"? Not the most convenient results when you sort by name. Whether this is the reason for the yyyy-mm-dd standard I don't know, but it's a reason I like it.

Besides the nice lexicographic order you get by default with YYYY-mm-dd, the timestamps are often extended with hour, minute, seconds, milliseconds. By starting with the year , adding them is done in a consistent way (see this).

ISO 8601 is intended to be "locale agnostic" and unambiguous, and this "biggest to smallest units" approach is IMO a good choice, with advantages already pointed by previous comments. It's an information exchange format, not a human friendly one (although it is friendly once you get used to it)

I am used to (European?) assortment of dates as with smallest to biggest and also most of my customers... I also think that smallest to biggest is the correct way to go from my personal experience as a teacher (always when I studied math it helped me).

What exactly do you mean by "I printed this HTML as output to my email box (via PHP)"? Do you actually mean that the form is submitted to a CGI script which e-mails the values to you, and the e-mail contains values in a format you didn't expect?

Date formats are specific to locale/culture/country/etc. What makes sense to one doesn't necessarily make to another (in Japan, the "normal" order is year-month-day, in USA, month-day-year, in Brazil, day/month/year, and so on - nobody is "wrong"). But ISO 8601 is not intended to be locale oriented, because it was designed to be "neutral" in that matter (as its focus is information exchange, specially between systems), so a standardized format that removes ambiguity and doesn't depend on cultural factors was preferred.

When handling dates in a system, one should prefer to send/receive in a standard format (to make parsing easier), and only when presenting this to users, a specific localized format is chosen - BTW, that's exactly what browsers (and probably your mail client) do: internally, the input value is in ISO 8601, but the date is displayed in a different format, according to user's locale.

@Peter Taylor, I meant that by the help of PHP, I create an HTML output from the HTML form. PHP processes my HTML form (after the client have filled in details and submitted it) and gives me / prints for me an HTML output as a new message in my local email client (Roundcube). While the client fills in dates which are in dd-mm-yyyy formats, the aforementioned output is (sadly for me) in yyyy-mm-dd format... But I want it to remain (or at least change to) dd-mm-yyyy format.