ISO 8601 defines a number of lesser known features. For example, it allows for not only fractional seconds (12:34:56.78) but also fractional hours (12.3) and minutes (12:34.93). There is a special notation for the midnight of certain day (24:00:00 or 24:00), and obviously another for a positive leap second (08:59:60 etc.). There are three ways to write a date: 2012-05-21, 2012-W21-1 and 2012-142. Intervals can be specified in terms of start and end, start and duration, duration and end, just a duration (i.e. no context). There are also recurring intervals which can be bounded in the number of recurrences or not. And so on and on and on.
That said, ISO 8601 tries to cover most cases for date/time representation. Implementing every bit of ISO 8601 is not desirable of course, but it is certainly worth looking at.
I don't see the need for a placeholder for the n-thousandth day of the year. Unless we want to accomodate something like 2011-0367 also equating to 2012-0002?
That said, ISO 8601 tries to cover most cases for date/time representation. Implementing every bit of ISO 8601 is not desirable of course, but it is certainly worth looking at.