Lupin and the Moon-a new line of thought

[gte510i]

No: HPFGUIDX 32995

For the Past week or two I have been reading up on the discussion of 
Lupin not turing into a werewolf until the cloud un-eclipsed the moon.
I've heard some interesting theories but I must admit that another 
issue precedes this.  
That is:  Lupin says that the wolfsbane potion need to be taken 
durring the week preceding the full moon.  Why did he need to take it 
that night?
It would seem that come nightfall he would have locked the door and 
curled up in his office as a harmless wolf.  He would have taken his 
potion the previous 7 days.  

Even if the full moon didn't show up until midnight, he wouldn't have 
turned into a monster, because tonight was not a 'take your potion 
night'.

Any thoughts?

catydid
who has had a lot of fun finding the idiosyncrosies in the cannon and 
thrilled that most have been addressed here.