es and son in telling time.
Sometimes I see es cuarto para las cuatro and sometimes i see son cinco para las tres. Why is it sometimes es and sometimes son?"
In this case, it is because "cuarto" means "a quarter" ("It is a quarter to four), so the verb is singular. Because "cinco" is plural ("it is five minutes of three), the verb is plural. There is a "Learn Spanish" lesson that will more thoroughly answer the basic use of es/son with telling time.
Because "cuarto" is singular (quarter), while "cinco" is short for "cinco minutos" which is plural.
I think it's the same for hours - es la una for one o'clock, but son las dos, tres, etc.