am 8 Uhr is wrong,
um 8 Uhr is correct. The prepositions in use with times of the day are
um = exactly that time
gegen = approximately that time
vor = before/in advance
nach = after
am is used instead of
um for dates e.g.
am 9. April.
um […] herum may also be used for expressing a loose time approximation (e.g.
Er wollte um den 10.7. herum mal vorbeikommen).
um can also be used in non-time-circumstances, e.g. as a local preposition or as demanded by a verb.