[lmm]

A string timestamp needs more documentation, because there are many subtly different string formats. Does it refer to a specific instant, or a political time notion? What separator is in use? Are leap seconds a possibility?

The ISO date format's notion of timezones is a compromise that gives you the worst of all worlds. They complicate referring to a physical instant, because you can refer to it in several timezones, rather than the unique representation of a unix timestamp. But they're inadequate for political time, because what time comes 6 months after 13:00 (+00:00)? (It could be 13:00 (+01:00) or 13:00 (+00:00) or likely others - you need a symbolic timezone like "Europe/Lisbon").

For physical or "system" times unix time is great (unless you need the greater precision, but how common is that?). For user-facing times ISO is inadequate. The use case for ISO string datetime formats is very narrow.

[gngeal]

"They complicate referring to a physical instant, because you can refer to it in several timezones, rather than the unique representation of a unix timestamp."

I thought it's supposed to be an external format. I'd always expect a computer system presenting the output information to the user in his local time zone, while accepting inputs from all time zones equally.

"But they're inadequate for political time, because what time comes 6 months after 13:00 (+00:00)? (It could be 13:00 (+01:00) or 13:00 (+00:00) or likely others - you need a symbolic timezone like "Europe/Lisbon")."

You can't standardize a changing practice. I'd never expect it to deal with these issues.

[lmm]

>I thought it's supposed to be an external format. I'd always expect a computer system presenting the output information to the user in his local time zone, while accepting inputs from all time zones equally.

We're talking about a web API here, not user display. But even so, IME users don't think of their timezones as "+8" or the like, so for human I/O you want to use symbolic timezone names, not offsets.