Sirius Black's math mistake?

[jferer]

No: HPFGUIDX 37848

Jamie:"When Sirius speaks to Peter (p. 370) he says, 
"Voldemort's been in hiding for fifteen years, they say he's half 
dead."  As far as I can tell, Voldemort can only have been in hiding 
for twelve to thirteen years, if the infant Harry defeated him on 
Halloween, 1981.  It may be just a rough estimation on Sirius's part, 
but why would he be so unspecific when he  clearly knows exactly how 
many years he spent in prison?"

It doesn't seem likely, but it may be that Sirius is counting some 
time before Voldemort was defeated as being "in hiding." The obvious 
objection to that is that Voldemort was at his most powerful before he 
came calling at Godric's Hollow.  Otherwise, I don't see an 
explanation.