Just checked the [RFC][1], but indeed I could not find anything about milliseconds. Only the `Z` suffix is part of the standard.

I think parsing the widely used format `yyyy-mm-ddThh:mm:ss.SSS[Z]` should be supported anyway. The fact, that browsers use this format (as stated [here][2]) should be reasons enough.

[1]: http://www.ietf.org/rfc/rfc3339.txt
[2]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toISOString