ISO 8601 and EDTF
There are many standards for encoding temporal data but the vast majority of these have been derived from ISO8601. The specification offers a variety of encoding options so it's rare to see an implementation of all of it. What most often happens is another standard adopts a stricter subset of ISO8601 rules, known as a "profile", and this is what is implemented in the wild.
The ISO8601 specification has been tightened up a bit over the years but very little has really changed. At the moment however the ISO committee are considering the first ever extension to the specification. I've been tracking a project called EDTF for a number of years. The Extended Date Time Format was developed by the Library of Congress to produce a reasonably comprehensive date/time definition for the bibliographic community. This project is currently before ISO and awaiting approval as an extension to ISO8601.