Jeffrey Wang wrote a new post about DAX Time Intelligence functions. This is an “under the cover” description that explains the rationale behind these functions, which need to be used following particular guidelines (described at the beginning of the post and annotated also in books on line).
Jeffrey explains really well why it is necessary having a date/time column as key column of the Calendar table you link to a fact table in PowerPivot. This is going to change in the future, supporting also the case of many existing data mart where the key column is an integer in the format YYYYMMDD.
There is also a priceless reference of the corresponding primitive functions used to implement some of the time-intelligence functions. Having this knowledge is important whenever you have to understand implications in complex calculations. There is also an interesting explanation about how the DateAdd function works and why it is necessary not having missing dates in the calendar table.
It is definitely a must-read post to learn DAX better!