First, remember to use a colon ( : ), not a period, when you write a time. The time you're asking about should be written this way: 6:45. (If you write 6.45, it is not a time. It is a number or quantity.) The time 6:45 is spoken this way: "six forty-five." (This is true in both US and British English.) The number 6.45 is spoken this way: "six point four five." (Again, this is just a number, not a time.)

Quarter to seven. Does that make sense?