Potters - A question

[Tanya Swaine]

No: HPFGUIDX 91353

In the book, Sirius says that he persuaded James and Lily to change secret 
keepers at the last minute and according to the lexicon, the charm was cast 
only several days before the attack.  Now, what I am trying to figure 
out.  Was Sirius ever the secret keeper under that spell?  Dumbledore 
didn't know about the switch that I can make out, so it must of been done 
just between James, Lily, Sirius and Peter.

However, this is my question.  At that time, Harry would of been 13-15 
months old.  The prophecy was overheard before his birth.  Surely it didn't 
take LV over a year to figure out who to target.  If James and Lily were 
not protected by that spell in the year or so leading up to the charm being 
cast with Peter as secret keeper, how did they remain safe for so long?

Tanya