Which function returns a correct ISO8601 date?

Hi, all

Some days ago, I needed a date, formatted as date("c") ...

To be a bit more object-oriented, I worked with an instance of
DateTime. My first thought was: "As the documenting for date() defines
'c' as ISO8601, I can take the constant provided in the DateTime
object ... right?" ... and that was wrong.

Here's a code-example:

date("c");
// 2013-09-07T12:40:25+00:00

date(DateTime::ISO8601);
// 2013-09-07T12:40:25+0000

-> Take special care to the notation of the timezone.

The method date("c") actually formats a date, fitting to the format
defined in the constant DateTime::ATOM.

Are both formats (with and without colon) valid for ISO8601, or is the
documentation for the method date() wrong?

Bye,
Simon