The tag URI scheme is a uniform resource identifier (URI) scheme for unique identifiers called tags, defined by RFC 4151 in October 2005.
This requirement makes each tag globally and persistently unique.
The authority name alone would not suffice for global uniqueness, since the ownership of domains and email addresses is subject to change.
The date may be abbreviated; the month and day default to 01, but tags which have defaulted month and day values are reckoned as different from tags where the 01 value is specified.
They all should have been minted by the same tagging entity (specifically, whoever controlled example.com on January 1, 2004 00:00 UTC).