Understanding about SQL index on DateTime Column
B-tree indexes don't work by comparing only for equality. They're designed (the algorithm is designed) to use ranges.

Figure 1.3 shows an index fragment to illustrate a search for the key “57”. The tree traversal starts at the root node on the left-hand side. Each entry is processed in ascending order until a value is greater than or equal to (&gt;=) the search term (57). In the figure it is the entry 83. The database follows the reference to the corresponding branch node and repeats the procedure until the tree traversal reaches a leaf node.

References

The Search Tree (B-Tree) Makes the Index Fast See especially section 1.3, B-Tree traversal
SQL Server index structure and concepts