It is not clear to me if ISO 8601-2004 (and hence, the lowest level of EDTF) provides for a way to express that a date/time with day precision (e.g. 2017-08-05) is known to be in a particular time zone (e.g. -04:00) without resorting to ranges. It seems to me this would occur very often, were a source only gives the date of an event, but the time zone in which the event can be firmly established.
For example, would 2017-08-05T-04:00 be valid?
I exclude ranges because many profiles of ISO-8601 do not support ranges.