An npm package's code is read as the codebase's own is: a constant computed with an export that, through everything it uses, reaches nothing and holds no state (ms, read from its code) is kept as a pure constant is; one computed with an export that reaches the clock (date-fns's `startOfToday`) isn't
A pure constant that computes something is evaluated once and kept: its module holds the value, and a run doesn't compute it again. One whose code reaches outside (the clock), writes out, or reads state something else could change first isn't: it's computed as written.
A test's result is kept for the runtime and test framework; one whose code reaches files (or the environment, or other processes) is kept for the platform too, which those differ by.