Can I look back at past years or plan future ones?

Yes. AnuCal shows 12 months at a time, and you can move that window. Switch years from the year picker, or slide the 12 months forward or back one month at a time: hover the month labels for the arrows, or press Cmd/Ctrl with an arrow key. Add Shift to move a whole year. Today, Reset, or Esc brings the saved view back.

By default you can reach last year through three years ahead. The calendar owner can widen that range under Year Range, in the Span section.