I'm not sure about the "fencepost error" part, but he's thinking of days as durations rather than points. It's early in the thread, about halfway down the first page:
> You don't start counting on sunday, it hasn't been a day yet, you don't start counting til monday. You can't count the day that it is, did you never take basic elementrary math?
Put in other terms, TheJosh uses "Sun - Sun" as inclusive start and exclusive end, while Justin-27 uses "Sun - Sat" as inclusive start and inclusive end.
I think TheJosh mixed things up when trying to explain it (durations vs inclusive/exclusive), so doubles down and comes up with weirder stuff later in the thread. I didn't read the whole thing though, stopped near the bottom of the first page.
