According to Wikipedia the date and time-format RFC3339/ISO 8601 usages includes:
On the Internet, the World Wide Web Consortium (W3C) uses ISO 8601 in defining a profile of the standard that restricts the supported date and time formats to reduce the chance of error and the complexity of software.
My specific need to format a System.DateTime-value in a RFC3339-format came when I used one of Google's REST APIs and they required a parameter in the RFC3339-format. Here is the code is the extension-method which provides the functionality of formatting a DateTime as a string in RFC3339-format:
public static string ToRfc3339String(this DateTime dateTime)
{
return dateTime.ToString("yyyy-MM-dd'T'HH:mm:ss.fffzzz", DateTimeFormatInfo.InvariantInfo);
}