The clock usually starts when a job requisition is approved or the vacancy is posted, and stops when the chosen candidate accepts the offer. Some organisations stop it at the start date instead, so the definition should be stated in any report. If a role is approved on 1 March and an offer is accepted on 12 April, time to fill is 42 days.
Time to fill reflects the whole process, including approvals, advertising, screening and hiring manager availability, so it is useful for planning and for spotting delays outside recruiting's control. Long vacancies cost lost output and overtime. Breaking the measure into stages shows where the days go, and screening often stands out: clearing applicants quickly against consistent criteria can save time without lowering the bar.