> "For time, is hour:minute:second sufficient, or do we need to support fraction of a second?"
In the library / museum / archive world, expressing something similar to 
"during the 1870'ies approximately" is often a more useful feature than a 
precision at the millisecond's level. This is one of the reasons why there 
was a need for EDTF and the reason why we focused on approximations (and 
other features) more than fractions of seconds.

If you consider it is useful (and it may probably be useful in some 
cases), it is OK for me to add this feature in EDTF. It is quite easy to 
modify the BNF for that and there should probably not be major problems to 
implement a parsing taking fractions of seconds into account. There are 
several more crucial (but also more difficult to describe and implement) 
features to be added to EDTF and I consider that fractional seconds is not 
a priority, but I consider it is OK to add this feature at level zero, 
remembering that it is part of xsd:dateTime to which I suggest to 
stick. Let's keep in mind though, that according to (page 16) ISO_8601 reads:

> "the decimal fraction shall be [...] the comma (,) or full stop (.). Of these, the comma is the preferred sign."


> "a decimal fraction of hour, minute or second may be included."

whereas xsd:dateTime only allows dots (full stops) as separators and only 
allows fractional seconds (not fractional hours nor fractional minutes).

This should not be a problem though, because EDTF level zero is defined as 
a profile of ISO_8601.