I have been tasked to review this document on behalf of the OPS DIR.  Overall, I struggled with this, and I don't know that "Has Issues" is the right result.  The document is well-written and clear (and let's face it, time is hard).  But as an operator, the backwards compatibility didn't resonate with me.  I realize there is a bit of chicken vs. egg here in terms of standardizing the new format before tooling will use it, but I think this could cause some incompatibility issues when you consider something like this:

```python
from datetime import datetime
datetime.fromisoformat("2022-07-08T00:14:07Z[!Europe/London]")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid isoformat string: '2022-07-08T00:14:07Z[!Europe/London]'
```

Until language libraries are updated for this, generators and consumers will be out of sync.  And let's face it: people don't always move to the latest version of their favorite language or libraries.

So unless I'm missing something (and I might very well be), I think this document would benefit from a section on migration guidance.  Moreover, what about some thought for some strptime/strftime format symbols for this to aim for language consistency?  I know this might not be the canonical place, but given the Java references it seems like a recommendation might be acceptable.

On the smaller side, I think Section 5 should be baked into Section 3 to introduce the u-ca notation before you use it in examples.