Clarify timestamp format

[pwinckles]

Someone recently opened a PR (pwinckles/rocfl#27) against rocfl and stated that UTC timestamps must be represented using a Z and not +00:00. I disagree with this, though I can see how the following text could be interpreted that way:

The value of this key is the datetime of creation of this version. It MUST be expressed in the Internet Date/Time Format defined by [RFC3339]. This format requires the inclusion of a timezone value or Z for UTC, and that the time component be granular to the second level (with optional fractional seconds).

In my interpretation, it just needs to be a valid rfc3339 timestamp, and that either UTC representation is fine as they are both valid under rfc3339.

Thoughts?

[zimeon]

My understanding (and I think intention) is that either Z or +00:00 may be used to indicate UTC in OCFL. This could be made clearer in the spec, maybe:

This format requires a time component granular to the second level (with optional fractional seconds) and the inclusion of either Z as an option to indicate UTC, or a timezone value.

(Curiosity: I note that RFC3339 defines slightly different semantics for -00:00 so that shouldn't be used for the timezone https://www.rfc-editor.org/rfc/rfc3339#section-4.3 )