My proposal is that referees ought to keep a calendar of time passed in game, preferably in a day planner, and use this instead of a traditional timeline or long form notes.
The value of a calendar is in keeping track of when things happened so that you, the referee, don't have to. When it comes time for the dying cultist to shout it "Our vengeance will come in X days", you can simply look up what X actually is, instead of having to retcon things two sessions later when it turns out X was supposed to be three instead of four. It also avoids all the little hassles of rectifying time, which are extremely common when the party splits up for any reason.
This is also one reason why I like the day planner, since it's broken down by hour or watch, so you can determine that between 9 and 10am on Tuesday May 31st, the villain will destroy the world. You may find that this level of precision actually helps you to effortlessly fill in details that otherwise weren't worth the work. It also helps provide colour - my experience is that most long campaigns take place over several years of game time. It can be tremendous fun to pull out the previous year's day planner and say "It was one year ago today you were impaled through the gut by your arch-enemy" or "The one year anniversary of your tower's construction is coming up". Birthdays, important events, festivals, upcoming plot points and world events, these can all go into the day planner, be totally forgotten by you, and simply referred to as the PCs progress through time.
Another advantage of the day planner is that the limitations on space discourage you from veering into long form note territory. The need to fit notes into the space pushes one to concise summaries, rather than verbose wastes of time.