adlnet/xAPI-Spec — pull request #561 by garemoko (2014-12-17 16:41:19+00:00)
Title: Clarify ISO 8601 duration

Fixes #477.

Note that the consensus between @fugu13 and @nashyura in the issue was that this requirement should be a MUST. I disagree for the following reasons:

* Russell mentioned that Wax supports the alternate format and has users using it. This is going to be a breaking change for those users.
* The alternative format is allowed under ISO 8601 so long as there is agreement between the communicating parties. On that basis, I don't see how an LRS can be non-conformant for receiving alternate format durations. It could be flagged as non-conformant for returning alternative format durations as presumably the test suite does not have prior agreement to accept that format. Equally, a test suite for content could brand content non-conformant for sending using that format.

I've used SHOULD* instead.