2.2.4 [W3C-XSD] Section 3.2.13, gDay
Since the lexical representation allows an optional time zone indicator, gDay values are partially ordered because it may not be possible to unequivocally determine the order of two values one of which has a time zone and the other does not. If gDay values are considered as periods of time, in an arbitrary month that has 31 days, the order relation on gDay values is the order relation on their starting instants. This is discussed in Order relation on dateTime (§126.96.36.199). See also Adding durations to dateTimes (§E). Pairs of gDay values with or without time zone indicators are totally ordered.
Indeterminate comparisons are considered to be determinate.