The exclude days option is a way to hide the number of days in your countdown timer and convert them into hours. It looks 23:59:59 visually looks better once you have counted down to less than 24 hours. 

The alternative would be to say Days: 0 Hours 23: Minutes 59: Seconds: 59. The exclude days option doesn't work if the number of remaining days in your timer is more than 1. We don’t convert multiple days into hours.