There are two calculated fields - Days to Assign and Days to Close. Those two fields are calculated based on the number of days it took for a specific action to happen.

Days to Assign is how long it took for a Lead to be assigned to a Sales Team (Assigned Date - Date Created).

Days to Close is how long it took for a Lead to be marked as "Won" or "Lost" (Won/Lost Date - Date Created).

Please, note, these fields will only be calculated if Leads are enabled in Configurations. Otherwise, the fields will always be equal to 0.