We know that Voldemort killed James and Lily Potter on the last day of October in 1981 (when Harry was 15 months old), as that is the date that appears on the memorial in chapter 17 of Deathly Hallows:
On this spot, on this night of 31 October 1981,
Lily and James Potter lost their lives.
We know that Professor Trelawney made the prophecy shortly before Harry was born, as per the description written on the prophecy in chapter 34 of Order of the Phoenix:
He stepped forward. Not as tall as Ron, he had to crane his neck to read the yellowish label affixed to the shelf right beneath the dusty glass ball. In spidery writing was written a date of some sixteen years previously, and below that:
This is confirmed by Dumbledore three chapters later:
"Who heard it?" asked Harry, though he thought he knew the answer already. "I did," said Dumbledore. "On a cold, wet night sixteen years ago, in a room above the bar at the Hog’s Head Inn.
This gives us about a year and a half between the prophecy and Voldemort's attack.
According to Sirius in chapter 19 of Prisoner of Azkaban, Pettigrew had already become a traitor a year before Voldemort's attack, i.e. shortly after Harry's birth:
"DON'T LIE!" bellowed Black. "YOU'D BEEN PASSING INFORMATION TO HIM FOR A YEAR BEFORE LILY AND JAMES DIED! YOU WERE HIS SPY!"
We know that Snape told Voldemort about the prophecy as soon as possible, from Dumbledore's statement in chapter 25 of Half-Blood Prince (my emphasis):
"Professor Snape made a terrible mistake. He was still in Lord Voldemort's employ on the night he heard the first half of Professor Trelawney's prophecy. Naturally, he hastened to tell his master what he had heard, for it concerned his master most deeply.
Seemingly, then, there was an entire year in which Harry was already already born, Voldemort already knew of the prophecy, and Pettigrew already was a traitor, yet in all that time Voldemort did not attack the Potters.
Why exactly did it take Voldemort an entire year to go attack the Potters?
Now this might depend on various other factors, so let's see what else we know. According to Fudge in chapter 10 of Prisoner of Azkaban:
"Not many people are aware that the Potters knew You-Know-Who was after them. Dumbledore, who was of course working tirelessly against You-Know-Who, had a number of useful spies. One of them tipped him off, and he alerted James and Lily at once. He advised them to go into hiding. Well, of course, You-Know-Who wasn't an easy person to hide from. Dumbledore told them that their best chance was the Fidelius Charm."
So it seems that as soon as Snape told Dumbledore that Voldemort was planning on going after the Potters, Dumbledore told the Potters to hide. But did they use the Fidelius Charm right away? The above wording implies that they did, but if so, Pettigrew (as the Secret-Keeper) could have reported their location to Voldemort immediately. If they hadn't used the Fidelius Charm at the beginning then Pettigrew certainly could have reported their location immediately. In the above mentioned conversation Fudge also said explicitly that Voldemort attacked the Potters barely a week after the Fidelius Charm had been performed:
"And then, barely a week after the Fidelius Charm had been performed —"
"Black betrayed them?" breathed Madam Rosmerta.
If that is correct then we have to wonder why they didn't use the Fidelius Charm until then, but more importantly, there was an entire year where the Potters were unprotected by the Fidelius Charm before Voldemort killed them.
So what was going on during that time that Voldemort couldn't attack the Potters for an entire year?
We might wish to suggest that Voldemort was waiting to get more information to confirm who the subject of the prophecy was, but that seems to be contradicted by Dumbledore's statement to Harry in chapter 37 of Order of the Phoenix:
"Why did he do it, then?" said Harry, who felt numb and cold. "Why did he try and kill me as a baby? He should have waited to see whether Neville or I looked more dangerous when we were older and tried to kill whoever it was then —"
"That might, indeed, have been the more practical course," said Dumbledore, "except that Voldemort’s information about the prophecy was incomplete. The Hog’s Head Inn, which Sibyll chose for its cheapness, has long attracted, shall we say, a more interesting clientele than the Three Broomsticks. As you and your friends found out to your cost, and I to mine that night, it is a place where it is never safe to assume you are not being overheard. Of course, I had not dreamed, when I set out to meet Sibyll Trelawney, that I would hear anything worth overhearing. My — our — one stroke of good fortune was that the eavesdropper was detected only a short way into the prophecy and thrown from the building."
"So he only heard... ?"
"He heard only the first part, the part foretelling the birth of a boy in July to parents who had thrice defied Voldemort. Consequently, he could not warn his master that to attack you would be to risk transferring power to you — again marking you as his equal. So Voldemort never knew that there might be danger in attacking you, that it might be wise to wait or to learn more. He did not know that you would have 'power the Dark Lord knows not' —"