Ser is used for dates and times (and events like the Olympics which move from country to country but the 2016 event will always have been in Rio).
Think of it as that moment, that day is always the same and does not change. What changes is the moment, or the day. For example, it is the 6th of March. That does not change. What changes is that the next day will be the 7th of March. But the 6th of March is still the 6th of March, and always will be.
Because in Portuguese "horas" is plural so requires the 3rd Person Plural conjugation: