The other responses are correct, but it's also worth pointing out that the 'a' before 'quarter' is optional - at least in British English. You can say either 'It's quarter to six' or 'It's a quarter to six'. Some people may consider it more correct and formal to use the article, but it is not a mistake to omit it. Personally, I have never used 'a' before 'quarter' : I have always said simply 'quarter to' and 'quarter past'.

The following are correct: "It's a quarter to 5." ('a' must be used here) "It's half past 8." ('a' cannot be used here) It is no different without "it's". It is always (to my knowledge) "a quarter..." and "half". Other examples: "It is a quarter to 5." "It is a quarter past 5." "It is half past 8." "It happened at a quarter to 5." "It happened at half past 8." "I will see you at a quarter to 5."

"Half past" but "a quater past/to".