What is the difference between um and am when used with times in German?
um 8 Uhr
am 8 Uhr
Am 8 Uhr is wrong, um 8 Uhr is correct. The prepositions in use with times of the day are
The preposition am is used instead of um for dates e.g. am 9. April.
Note that um […] herum may also be used for expressing a loose time approximation (e.g. “Er wollte um den 10.7. herum mal vorbeikommen”).
Besides, um can also be used in non-time-circumstances, e.g. as a local preposition or as demanded by a verb.