I would take a guess that the meaning is У вас день проходит так медленно, как будто год? ("Is your day going so slowly that it seems like a year?") In other words, when we are very bored, or in great pain, or otherwise in unpleasant circumstances, the time seems to pass more slowly. And I would interpret the use of за + accusative to be similar, in this context, to Она приняла резиновую игрушку за настоящую змею ("She believed that the rubber toy was a real snake.") or Шпион выдавал себя за учителя ("The spy passed himself off as a schoolteacher.")

Is this an expression you heard in a song, or what?