Does your janky string format of “18-03-2024” suddenly has to become aware of the timezone

No, there is no timezone, and that is the entire point. In the majority of cases you just want to store the local date. The point is that a local date or time is not necessarily a fixed point in time. If I have drinks at 18:00 every Friday, that doesn't change when we switch to or from DST, it's still 18:00 in local time. I don't need a timezone, I know what timezone I live in.

Now, in cases where timezones do matter, for example if you have a Zoom meeting with someone from another country, you can store as local time + timezone. But this is still very different from storing a Unix timestamp. This meeting will be at a specific time in a specific timezone, and the exact moment in time will adjust when changes are made to that timezone. Again, a Unix timestamp does not allow for this, as it's always UTC.

I assure you that 1710720000 will translate to the same janky “18-03-2024” format you’re using every single time unless you deliberately mess with timezones in code

No, it doesn't. You can't convert it to any date unless you "mess with timezones", because 1710720000 is a specific moment in time and you have to provide a timezone when converting it to a date. You are mistaking the fact that some systems implicitly use UTC when converting for some sort of of universal standard, because it's not.

Run the following Swift code:

let d = Date(timeIntervalSince1970: 1710720000)
print(d.formatted(date: .complete, time: .omitted))

You'll get a different date depending on your location.

If your code will break because someone has different OS settings than yours, you are writing bad code.

Yes, and your bad code will break simply because you are abusing a datatype for something beyond it's intended use. If you want to store an absolute point in time, by all means use a Unix timestamp, but if you want to store a local time you should never use it because it's not mean for that and it doesn't encode the information needed to represent a local time.

Even this would be better than a string:

struct { int year byte month byte day }

Yes that's fine. I'm not arguing that you should store it as a string, I'm arguing that you should store it as individual components, in whatever format, instead of seconds since the epoch. As long as the format is well specced it doesn't really matter. Strings are used all the time for representing dates by the way. For example, ASN.1, which is used everywhere, stores dates and time as strings and it's perfectly fine as the format is specified unambigiously.

Six bytes as opposed to 10

In what archaic system are int's still 4 bytes? Int is 64-bits, or 8 bytes, on any modern machine. If I read your format on a 64-bit machine, it'll break. Also is that int little or big endian? You code still breaks if you spec an int32 and you store your date on an x86 machine (little endian) and I read it on a big-endian machine. You know what's not ambiguous ? "This time is stored as an ISO8601 string".