As I remember, 5:30 is American and 5.30 is British.

I would always use a ":" and never a "."

Always " : " " . " is really for money People will understand if they have asked you for the time though so don't panic.

You specify ¨saying it¨ not writing it ... so saying it we say 18:00 as eighteen hundred 18:00 - "sixteen hundred" 18:45 - "sixteen forty-five" 18:05 - "sixteen oh-five" Military tend to say eighteen hundred hours. In real life, we don´t tend to speak in 24hr time so if saying it we would normally convert it to the 12 hrs clock and just say 6 pm. Now if you wanted to know how we write it we should always write it with the ¨: ¨ as the one dot is for currency. However, sometimes, you will see it with only the one dot but the context it is written in will tell you it relates to time, not money.

Always " : " " . " is really for money People will understand if they have asked you for the time though so don't panic.