Please clarify the meaning of filter functions applied to DateTimeOffset values

Consider for example the 'day' function applied to a DateTimeOffset.

Is this supposed to return the 'day' of the value when it is considered as UTC, or the 'day' component as it appears in the original literal value, which implies that the zone offset must be preserved when DateTimeOffset values are propagated?

If DateTimeOffset values are permitted to be 'normalized' by agents, but are not required to be, then the 'day' function applied to DateTimeOffset value would appear to be meaningless, unless it returns the UTC 'day'.