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The T and the Z in RFC3339 aren't at the same position in the date/time string, so I think there's some misunderstanding here about their roles, either in the summary you've read or in your interpretation of it.

The example given in RFC3339 is "1985-04-12T23:20:50.52Z", where T is a separator between date and time (nothing to do with time zones, just another separator like : and -), and Z is a shorthand for +00:00, denoting no offset from UTC (and it's UTC not GMT, at least according to the RFC text).

My understanding of ISO 8601 is that its way to write "12th April 1985 at 23:20:50.52 in a time zone I am not specifying here" is to remove the time zone designator entirely, like "1985-04-12T23:20:50.52". RFC3339 specifically doesn't allow an unspecified time zone, though.

The Z is borrowed from an older military standard where times are suffixed with a single letter representing the time zone (spoken using the NATO phonetic alphabet, hence "Zulu time" for UTC), but in RFC3339 the only time zone where this shorthand is allowed is Z, and all the others need to be spelled out as +05:00 or similar (so in particular T is not a valid timezone suffix in RFC3339). As far as I know, the same is true for ISO 8601.