Yes, the granularity question is key. Even if you are stamping your datetime field with getdate() then its possible though unlikely you could get duplicates.

It sound like you have two questions:

1) What is the proper key for the table? If you show us your table structure and describe its use we could help identify what the key should be or you could use the identity feature to create an artificial key.

2) When presenting the data from my table how can I generate a daily sequential number corresponding to the time the record was inserted.