Refactor and harden date normalization in data layer.

Change date normalizer helper to an instance
method. Add a convenience on top of it. Refactor
existing calls.

Minor reorganization. Minor hardening.
Fix title view updating on landscape Months screen.

Properly update viewportYOffset given status bar
changes after auto-rotation.
Refine title view updating on landscape Months screen.

Make more accurate by factoring in month header
label's top margin. Make sure to cache it to avoid
constantly dequeuing header views.