How does indexes on a date field work?
SQL Server non-clustered indexes are sorted in ascending order by default, unless you specify that they must be in descending order, when you define them.

(Also worth noting that if a table is a clustered index, that clustered index will be included in all non-clustered indexes as well.)

When it uses that index, it will usually do a seek into the index at the correct spot, using the index's B-tree structure, otherwise it will scan the entire index if the index doesn't satisfy the query.