几年如一日means time goes by so fast.or means somebody insist on  doing  sth. do it for several years and every day as if it were the first day. In fact ，when it with the second meaning， people can also use 几十年(Several decades)如一日or 十几年(more than ten years)如一日
People usually use it to describe somebody is persevere. For example:他每天坚持跑步，几十年如一日。(He instead on running for decades and do it every day as if it was the first day.)