[nophunphil]

One factor complicates things a bit: this is one way to encode that the timezone is UTC, but +00:00 is also commonly used for example.

RFC 3339 is nice for this reason. Always UTC and terminated with a Z.

[GoblinSlayer]

RFC 3339 isn't always UTC and doesn't mandate Z, in only removes some extra flexibility of ISO 8601, like comma separator or short syntax.