Hej, i got two examples here of time, the first one is dativ used (dögum):

Þat var á dögum Haralds konungs ins hárfagra, Hálfdanar sonar ins svarta, at sá maðr kom skipi sínu til Íslands í Breiðdal, er Hallfreðr hét.


and the second one is accusative used (nótt):

Ok eina nótt dreymdi hann, at maðr kom at honum ok mælti: "þar liggr þú, Hallfreðr, ok heldr óvarliga. Fœr þú á brott bú þitt ok vestr yfir Lagarfljót. Þar er heill þín öll,"


Which one scould i use? :s

Is the dögum in dative only becuase of á?

Well I dont study at university, so a detailed plain word explaination would be nice. but of course I'll be happy for alle the help i can get :b

thanks