And I wondered the exact same thing as well!
It's pretty clear from context what is meant. Since it's taking about eating, it's likely that hours is meant, and not days, or even floors.
We have to use some assumptions in English, too. If I said "We will have dinner on the 20th" it could mean April 20th, May 20th, the 20th of some other agreed-upon month, or even the 20th floor of a building.
In English, it doesn't necessarily have exactly one meaning when it's taken out-of-context, and neither does it in Esperanto.