I don’t know which “epics” you’re referring to, but I really don’t think there’s “a way.” There are multiple, even within a single book.
The Cloud Atlas has 6 distinct periods across which the 6 stories unfold, and I couldn’t for the world of me remember what the Mitchell did, had to look it up. The first story is told via diary entries, with date stamps, the second is letters, there’s one that’s in the form of an interview, etc. (note that I’m not implying this is a great example, but it’s an example).
I agree with what Vern said though. How important is it for the reader to know the exact time passage? What works for your story?
—she returned there a week later
—His hands grew calloused from months of work aboard the ship
—Over the years, Cercei’s drinking habit made her too fat for her dresses
—mentions of seasons, birthdays, anniversaries
—a change of setting already an indicator of time passing, e.g. if he was a uni student and now he’s working in an office. Dialogue can give more details and colleagues always ask how long you’ve been working there
—And if you really hate your readers: “after exactly 18,753 hours the ship entered the Bla solar system.” But things do get really confusing between different planets with different calendars. It’s not usually addressed very well (or at all).