|analogue||digital||normal use||timetables, TV, etc.|
|6:00||It's 6 o'clock.|
and the previous hour|
|It's six oh five.|
|6:15||It's quarter6.||It's six fifteen.|
|6:30||It's half6.||It's six thirty.|
|6:35||It's 25 to 7.||to and the following hour
|It's six thirty-five.|
|6:45||It's quarter to seven.||It's six forty-five.|
|6:55||It's 5 to 7.||It's six fifty-five.|
1) In English there is no 24-hour-system (sometimes it is used on TV and in timetables).
15.00 is 3 o'clock (p.m.) (on TV: fifteenhundred). That's why you use a.m. (Latin: ante meridiem) and p.m. (post meridiem) in situations where it is necessary to point out that you want to say "in the morning" or "in the evening". In other situations (or when it is clear that school starts in the morning and the party in the evening) it is left out.