It's a quarter to seventeen(sixteen forty- five )now. It was sixteen thirty (half past sixteen)fifteen minutes ago. It was sixteen fifteen(a quarter past sixteen)thirty minutes ago.
It is quarter to seven now (or 'It is six forty-five now' or, in the evening, 'It is 18.45)
It WAS (you need the past tense, 'It's' is present tense) six-thirty (or half past six' or, in the evening '18.30') fifteen minutes ago .
It WAS six-fifteen (or 'quarter past six' or, in the evening, 18.15') thirty minutes ago.
Confusing isn't it? Well done.