When do werewolves change?

[Mark D.]

No: HPFGUIDX 53899

OK, found a critical flaw in Prisoner of Azkaban. Professor Lupin 
does not change into a werewolf until the group emerges from the 
tunnel from the Shrieking Shack. When the clouds part and he sees 
moonlight Lupin morphs. 
Imagine my horror when I thought I found a mistake in JKR's logic 
far worse than the order of Harry's parents emerging from the wands 
in GoF.
Dumbledore sent Harry and Hermione back at 11:55 "It is five minutes 
to midnight. Miss Granger, three turns should do it." 
This would place Lupin's transformation somewhere around 9:30 to 10.
Moonrise of the full moon is at 6pm with the moon reaching its 
zenith at 12 and setting at 6 am. Everybody knows that! Lupin should 
have been a werewolf a full three hours before Harry and Hermione 
went back in time!

If a werewolf only changes from moonlight, all Lupin really needed 
to do to prevent his change would be to stay in a windowless room. 
That would eliminate an important storyline on PoA and the whole 
tapestry of JKR's Harry Potter begins to unravel... or does it?
I have two theories, See what you think
1) Moonrise at Hogwart's is later. Remember it is against mountains 
and moonrise might well be up to three hours later. or...
2) Werewolves stay werewolves for the full three days of the full 
moon, not just moon rise to moonset. 
If you check moon phase charts (as Lupin asked Hermione if she had 
done) the full moon starts at a certain hour and minute. I'm 
suggesting that Lupin DID NOT CHANGE because the moon came out from 
behind the clouds, but because full moon started at that point.

Personally, I don't buy the Harry Potter Lexicon's stand of placing 
the books in the early 1990's. It places PofA in 94-95 which the 
climactic night of Harry meeting Sirius on June 6 1995. The great 
flaw with trying to tie a fictional book down to a physical date is 
that JUNE 6 1995 WAS NOT A FULL MOON!!!! According to the Phases of 
the moon table, the full moon for June 1995 was on the 26th starting 
at 11:33 am GMT!

Does this make any sense to anyone else? Opinions?