I'd consider using both. A date picker to go to specific dates and semantic zoom to switch from daily to weekly, etc. They both serve different purposes. I'd consider putting the date picker in an AppBar as a go-to-date command rather than using full-time
real estate.
Semantic zoom is typically two level. Expanding the concept beyond that makes sense for the calendar scenario, but make sure not to make it too cumbersome to move between common levels. You will need to implement the many level version yourself rather than
using the built in semantic zoom controls.
--Rob