I haven't run into that use of -0000, that's insane. I suspect a side-effect of libraries lacking support for the full range of ISO8601 (for example, refusing

I haven't run into that use of -0000, that's insane. I suspect a side-effect of libraries lacking support for the full range of ISO8601 (for example, refusing to parse a value without an offset, forcing people to use such hacks). Wouldn't surprise me; iOS doesn't even have a consistent way to parse ISO8601 values with and without milliseconds. [edit: Apparently this is a RFC3339 thing, did not know that: https://tools.ietf.org/html/rfc3339#section-4.3 https://tools.ietf.org/html/rfc3339#section-4.3 Negative zero offset is invalid in ISO8601, but valid and carries the meaning you describe in RFC3339. I misread what that meaning actually is from your comment - it is different to a "floating" or unqualified local time, it always represents a UTC time but with an unknown local time offset. ]