The accusative case is used for definite time expressions without a preposition, e.g.

             masculine: der Tag -> jeden Tag (every day)

             feminine: die Woche -> nächste Woche (next week)

             neuter: das Jahr -> dieses Jahr (this year)
 

in einem Jahr (in one year) or vor einer Woche (one week ago) are examples of  definite time expressions with a preposition; then the preposition determines the case.