Time to fill measures the days between a position opening and its fill date under the definition used here. It belongs to Position. Application-to-hire duration belongs to Job Application and answers a different question. Include positions filled within the reporting period, even if they opened earlier.
An open position has no completed fill duration. Leave its value blank and exclude it from the completed-position average. Averaging the elapsed age of vacancies into that measure would mix completed and unfinished work. Calculate each filled position's days, then average those durations without weighting by candidate count. The exercises also ask for a period without fills; this simulator returns zero for that empty aggregate, while the detail report retains no completed records for the window.