The incubation period, or the time interval from infection to onset of symptoms, is from 2 to 21 days. The patients become contagious once they begin to show symptoms. They are not contagious during the incubation period.
I don’t believe that for a second. In my many years of nursing, the standard was that with a virus, one starts becoming infectious within the 24 hours prior to onset of symptoms. I don’t think they want that info spread because it would make people panic.