Why does a Z suffix RFC 3339 timestamps result in a time zone annotation of Etc/Unknown?

[nyarly]

I'm finding that biff treats times in the format 2025-10-15T17:48:50Z as being in Etc/Unknown. I would have expected the Z suffix to specify UTC, as per https://datatracker.ietf.org/doc/html/rfc3339#section-5.6 .

I'm aware that time zones and time representations are fraught, but I was wondering why Buff behaves this way?

That said, it's an invaluable tool!

[BurntSushi]

Your link to RFC 3339 isn't quite right. The specific section you want is RFC 3339 Section 4.3:

If the time in UTC is known, but the offset to local time is
unknown, this can be represented with an offset of "-00:00". This
differs semantically from an offset of "Z" or "+00:00", which
imply that UTC is the preferred reference point for the specified
time.

That is, Z is equivalent to +00:00 in meaning that UTC is the preferred reference point. According to this section, you are correct that 2025-10-15T17:48:50Z should parse with a time zone annotation of UTC.

However, RFC 9557 Section 2.2 revised this part of RFC 3339 to read as this instead:

If the time in UTC is known, but the offset to local time is
unknown, this can be represented with an offset of "Z". (The
original version of this specification provided -00:00 for this
purpose, which is not allowed by [ISO8601:2000] and therefore is
less interoperable; Section 3.3 of [RFC5322] describes a related
convention for email, which does not have this problem). This
differs semantically from an offset of +00:00, which implies that
UTC is the preferred reference point for the specified time.

In other words, Z is the most widely supported way to indicate that the preferred offset from UTC is not known. RFC 3339 diverged from ISO 8601 on this point. RFC 9557 corrected that.

Note that Biff supports -00:00 too, and treats it as equivalent to Z:

$ biff time parse -f rfc3339 2025-10-15T17:48:50Z
2025-10-15T17:48:50Z[Etc/Unknown]

$ biff time parse -f rfc3339 2025-10-15T17:48:50-00:00
2025-10-15T17:48:50Z[Etc/Unknown]

$ biff time parse -f rfc3339 2025-10-15T17:48:50+00:00
2025-10-15T17:48:50+00:00[UTC]