Q. I have been under the impression that extensions on a date (st, nd, rd, etc.) are proper when used simply with a month (January 15th) but are not used in connection with a year (January 15, 2009). Please advise if this is correct or provide instruction to the contrary.
A. Chicago style doesn’t include the extensions in either case. We limit ordinals to a day appearing alone or ahead of the month (the 5th of May; see CMOS 9.33). That said, the style that you describe is common and not incorrect.