Date Constructs in the Atom Syndication Format

One of the questions that has come up is why RFC 3339 style dates were chosen for the date format in Atom. The simple answer is that any date that is conformant to RFC 3339 is also conformant to XML Schema, ISO 8601, and W3C Datetimes.

The slightly longer answer is that all the other schemas are more complex than RFC 3339. In particular the daddy of them all, ISO 8601 is the most complex. For example, all of the following are valid ISO 8601 dates:

1997
1997-07
19970716
1997-07-16 19:20+01:00
1997-07-16T19:20+01:00
2003-W14-2
2003W142
12:30:01
19

Care to guess what that last one represents?

Umm ... A really, really old unixtime, i.e. 19 seconds after the "UNIX Epoch" began?

Posted by Joe Grossberg on 2005-08-18

To make that wonderful interop happen, we had to profile RFC3339 just slightly:

"In addition, an uppercase "T" character MUST be used to separate date and time, and an uppercase "Z" character MUST be present in the absence of a numeric time zone offset."

Posted by Robert Sayr:e on 2005-08-18