So this is in a WHERE clause ??

Assuming so, this is what is known as a non-sargable condition.

Basically, as you have wrapped the datetime column in a function then SQL Server has to look at and process every row rather than use the more optimal route of an index.

Definition :

http://en.wikipedia.org/wiki/Sargable

One of the many blog articles available:

http://beyondrelational.com/modules/2/blogs/66/posts/9925/sargable-predicates.aspx