The one big failure of RFC 3339 is that it doesn't specify the actual rules for the separator: Applications using this syntax may choose, for the sake of

The one big failure of RFC 3339 is that it doesn't specify the actual rules for the separator: Applications using this syntax may choose, for the sake of readability, to specify a full-date and full-time separated by (say) a space character. Is separating it by a tab valid? A tilde? An underscore, as this website shows? Since this is so underspecified, there will be receivers that accept only T, and others that accept the T and some arbitrarily chosen character set. The ABNF only allows T. I don't know how a standard that was specifically written to resolve the issues resulting from such ambiguities ended up with such a crucial issue.