Time prepositions
Use um for clock times, am for days and parts of the day (except in der Nacht), and im for months and seasons. Seit describes an ongoing period; vor places an event in the past; in can mean a period from now. These time uses of seit, vor and in take dative.
Understand the idea
German time expressions use different prepositions for clock times, days, months, durations and starting points. Their meanings overlap with English prepositions only partly. The distinction between a continuing duration and a future delay is especially useful.
Build it step by step
Use um for clock times, am for days and many parts of a day, and im for months or seasons. In can introduce a time from now; seit marks a starting point or duration continuing to the reference time. Vor can describe how long ago something happened. After dative time prepositions, the article and sometimes the noun need the appropriate ending.
See it in a sentence
Der Kurs beginnt um acht Uhr.
Wir treffen uns am Montag.
A useful memory aid
Contrast in zwei Tagen with seit zwei Tagen: one points forward from now, the other describes a duration that has already begun.
A common trap
Remember that halb sechs is half an hour before six, so it means 5:30. Also learn in der Nacht separately rather than extending am to every part of the day.