Time passes for characters in the game just like it does in the real world: seconds, minutes, days, years, etc. that proceed in linear order. However, much like a video game or a movie, time for your character typically doesn’t perfectly line up with the real-world. Days and years in-game will often pass in a real-world instant, while crucial seconds in the game world may take considerably longer in the real world.
While neither of these terms are used in the official rules text, it’s helpful to think of two methods of tracking: Narrative and Combat.
Narrative time is often abstract, and is used when players are performing a task which takes while (more than a minute), but the specifics of the task typically don’t require narrative attention. Traveling, sleeping, eating, and other mundane, routine tasks are great examples. Sometimes these tasks will take fixed amounts of time (8 hours for a long rest, 1 minute to descend an elevator, etc.), but in other cases the Dungeon Master may need to roughly estimate how long a task takes.
How many minutes a task takes typically doesn’t matter unless the players are under some kind of pressure (I’m in a house that’s on fire, maybe I shouldn’t spend 5 minutes reading this letter), so a rough estimate is usually fine. If you have a rough idea of the hours passing throughout a day you’re usually fine. Tracking longer periods, such as days, can be more important. If events are happening in the world, tracking the passage of days helps to track when those evens occur. It can also be fun to look back on the weeks, months, and years of a character’s career and see where they started and how far they’ve come.
For more on Narrative Time, see pages 32-33 of the Dungeon Master’s Guide under “Tracking Time“.
Combat time is strictly measured in 6-second “rounds”. During a single round, every creature participating in the combat has a “turn” in which they act, each occuring in order based on the creatures’ initiative results. Creatures continue to act in this order until combat ends. Regardless of the number of creatures acting, rounds always take 6 seconds and turns occur one by one. While this may defy real-world logic, it’s an effective abstraction of the real-world.
More on rounds, turns, and initiative will be covered in the Combat section of this guide.