When we report the AQI for a day that has already happened, we are reporting the AQI for the last 24-hour time period.
Particle pollution has a national air quality standard based on 24 hours, so we take the value for each hour from midnight to midnight and average them together to get one AQI for the day for particle pollution.
For ozone, the national air quality standard is based on 8 hours. We calculate 17 overlapping 8-hour periods. The first one starts at 7am and goes to 3pm. The second 8-hour average is from 8am to 4pm, then 9am to 5pm and so on. The last few 8-hour averages actually include hours from the next day. The last 8 hours are from 11pm to 7am. The highest of those 17 8-hour averages becomes the ozone AQI for that day.
Whichever AQI for the day is higher (the AQI for ozone or the AQI for particle pollution) is the overall AQI for the day.