More of an avoidance than a hard-and-fast rule. As an ordering method, not a problem. As a clustered index, ok. I'm a little concerned about how a clustered datetime would affect multiple users inserting at the same time. But try querying for 2/16/09 8:17pm (the time right now MST), in a table where the datetime is acting like a timestamp. It's more like indexing a real number.

As for high selectivity, I DO index GUIDs. But I would never make a user enter one!

Yes, dates are integers at some level. But my users' concept of dates don't extend to 3.333 milliseconds.

Now, confession time. I actually LIKE Oracle's approach of storing the time as the decimal portion of a floating point number, where it's the fraction of the day. Maybe that's where the aversion for indexing dates comes from.

Kevin