One of the hardest things to keep track of, whether it’s in a story or an RPG, is time—where it’s going and how much of it has gone by. Having clocks to check and days to count can be hard enough (says a person who mainly remembers what day of the week and month it is by her watch and whether she’s at work), but some situations—major league cram sessions, hospital stays, prolonged healing trances, trips through outer space or the Underworld, stays in time-dilatory planes of existence, relative boredom, what have you—it’s hard to measure only in how many times the sun’s set and risen. So how else can you look at time passing?
General times of day, and their corresponding light levels and temperatures. Admittedly, artificial lighting and heaters/AC have let many of us ignore this part of how the world works, but it’s still there. The sun comes through differently at different times of day; the shadows fall in different places. It’s not unusual for me to sit down for a Tuesday game session with sunlight outside my window and pull myself out of it with light shining straight into my eyes.
Sky-time. Most of us are used to the phases of the moon, and at least have some vague idea how that four week-ish lunar cycle thing works. For longer-term periods (or for someone falling asleep outdoors while stargazing) there’s also the cycling of the stars through the sky.
Regular landmarks. Does the courier come by every three days? Is there worship at regular intervals? Have some chores been settled into a weekly rotation? If you’ve got a pattern, having your time-jump start at one point in the pattern and end at another should get the point across.
Weather and seasons. I don’t just mean “oh, it’s winter, it’s bitterly cold and snowing” or “hey, it’s summer, prepare to fry”, though those work decently well in the long term (as long as you know your climate, anyway). If you want to have a few days go by, smaller weather patterns might be the way to go—a brief round of showers, a particular heat wave, the time it takes the remnants of one evening’s snowfall to melt away, the time it takes clouds that look borrowed from another climate’s sky to leave. Granted, weather being weather, these are neither predictable intervals nor well-known enough to give people exact time, but they’ll certainly show that time has passed.
Things that take a known (or reasonably estimable) period of time. You might look at something like fruit spoiling, or flowers going from bud to blossom, or hummingbirds emptying out their feeder; at the time it takes for milk to go bad, or for that dead rodent on the walk everyone’s too squeamish to deal with to be carried off by something larger and with less exacting taste.
Using clues like this may not get time across perfectly, but they’ll definitely make it clear that the time isn’t just standing still waiting for narrative convenience.