A long-duration experiment starts at 9 a.m. on the circled date and runs for exactly 1000 hours. On which day and date does it end?

1000 h = 41 days 16 h. 4 October + 41 days = 14 November (27 days to 31 October, then 14), a Sunday (41 = 5 weeks + 6 days, Monday + 6). Adding 16 h to 9 a.m. gives 1 a.m. of the next day: Monday, 15 November. (Sunday, 14 November forgets the 16 hours.)