Asking
Jam berapa? ‘What time is it?’, literally ‘hour how much’.
On the hour
Jam tujuh. ‘Seven o’clock.’
Past and to
Jam tujuh lewat sepuluh. ‘7:10’, literally ‘seven past ten’.
Jam tujuh kurang sepuluh. ‘6:50’, literally ‘seven less ten’.
The half-hour trap
Setengah tiga is 2:30, not 3:30.
Indonesian counts the half as belonging to the hour it is heading towards: half of the third hour. German and Dutch do the same thing, which is probably where it came from. English speakers read it as “half past three” and arrive an hour late.
If you are unsure, say jam dua tiga puluh ‘2:30’ and avoid the whole problem.
Adding the part of day
The 24-hour clock exists but conversation uses the day-part words from the greetings lesson.
Jam tujuh pagi ‘7am’
Jam tujuh malam ‘7pm’
Jam dua siang ‘2pm’
Jam empat sore ‘4pm’
Without one of these, a bare jam tujuh is ambiguous, and Indonesians will normally add it automatically.
Duration versus clock time
Jam is the clock. For a length of time, use jam with a number and no jam prefix confusion:
Dua jam. ‘Two hours.’
Jam dua. ‘Two o’clock.’
Word order alone carries the difference.