That’s true in the Parallel Universe, too, where time expands and contracts for characters experiencing fictional life, as well as for their author (as narrator), and for readers (as observers). Writers who are pantsers don’t do much, if any, planning, plotting, or outlining when writing, but keeping an eye on the passage of time is important no matter how much or little of it elapses during the story.
For a historical novel set sometime on Earth, in addition to coordinating the plot with important dates, a writer may need to know the phases of the moon, and a calendar will be important for determining the impact of the weather on fictional events as well as the historical ones. Where in time do you need to go? 1066? 1776? 1871? 1914-1918? 1939-1945? A perpetual calendar is the place to start.
The website timeanddate.com offers one that can be customized and printed (PDF). Once you have the information provided by a perpetual calendar, it’s easy to construct your story’s fictional calendar, and then use it to verify continuity when you’re editing.
Time is important no matter what the genre, so if your story involves other-world-building, you’ll need to come up with a method of reckoning based on your planet’s periods of rotation and revolution. How much light, by how many suns, constitutes daytime? Is there more than one moon to illuminate the night? Do the inhabitants of your world use constellations to reckon longer periods of time? Do they use clocks and calendars driven by radioactive decay?
Yes, indeed, time flies, but you can put away your stopwatch, because unless you’re writing a story like The Poor Little Rich Girl (which, among other more ghastly things, explores bizarre alternative meanings for common idiomatic expressions), you probably don’t need it for racing insects.