It is an idiom. I don't know why Duo does this, unless it likes to frustrate people.
Actually, I'm not so sure it is an idiom. It could be in some instances, yes, but if you live in Canada or The United States, there is a period in all weekdays between either 5 and 7 in the morning or 4 and 7 in the evening when everyone is getting back from work and school, which is called Rush Hour. :)