Creating a index on a date column that's already populated
There is no downside in creating an index at a later moment. The index will perform the same way like it would when it was created when the table was created.

You mentioned that the table has already 10 million rows. Therefore I would suggest that you create it during a 'calm' period. This because Oracle needs to lock the table. This cannot be done if there is a transaction that does an insert, update, or delete in the table.