Chicago is typically 15 hours behind Seoul. When it's 11:00 PM in Seoul, it's 8:00 AM the same day in Chicago. South Korea does not observe daylight saving time, but Chicago does.
During US daylight saving time (March to November), the difference is 14 hours. During US standard time (November to March), the difference increases to 15 hours.
Optimal meeting window: 5:00 AM - 7:00 AM Chicago time (7:00 PM - 9:00 PM Seoul, or 8:00 PM - 10:00 PM in winter). Very early morning Chicago catches evening Seoul.
Chicago has a significant Korean-American community, particularly in the Albany Park neighborhood. Korean Air operates direct flights from O'Hare to Seoul.