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yea. A day or shorter is В +accusative

a week = На + prep

and i guess a year is в plus prep

No wrong.

В этот день, в семь часов.

Well that's the thing. When talking about weeks, then its на + locative

На прошлой неделе - Last week, etc.

Well, there's ways around that.

there is a song called Надежда на завтра

Yep...