From the Wikipedia page of the Novel-

Pat believes he has only been away a few months, but soon realizes it has been years, and struggles to piece together his lost memories and what has become of his wife, Nikki. He has a theory that life is a film created by God and that its "silver lining" will be the end of 'Apart Time' with Nikki.

So this lines show the real reason of naming the novel this way and movie just copied the title, skipping the explanation.

From the Wikipedia page of the Novel-

Pat believes he has only been away a few months, but soon realizes it has been years, and struggles to piece together his lost memories and what has become of his wife, Nikki. He has a theory that life is a film created by God and that its "silver lining" will be the end of 'Apart Time' with Nikki.

So this line shows the real intent of naming the novel this way and the movie just copied the title.