It maybe easier to think of it like a 24hr clock.
Cycle starts at midnight, IVO happens at mid day, inlet closes at 8pm, EOI needs to be around 10.30am, 9.5hrs before 8pm.
If your IVO earlier at 11am, you need to advance EOI earlier, 10.5 hrs earlier instead of 9.5hrs, before 8pm.
The boundary is at 8pm.
If it a gen 3, boundary is still 8pm, EOI is at 10.30am the following day, 14.5hrs later or 13.5hrs later if IVO happens at 11.00am instead of 12pm