After rolling lots of UTC Date data up through the partner API I was initially puzzled why a user on PDT (UTC-8) would be seeing the same value in the Salesforce interface as a user on NZDT (UTC+13).
In hindsight this makes sense, a Date without time doesn't really have a sensible adjustment for UTC to the users timezone.
From Primitive Data Types:
Date data. Fields of this type contain date values, such as ActivityDate in the Event object. Unlike dateTime fields, date fields contain no time value—the time portion of a date field is not relevant and is always set to midnight in the Coordinated Universal Time (UTC) time zone.