The PP is correct - you get 90 days on the new device, or the remainder of your 1 year. We just went thru this with DD's Nano - trying to decide if we were going to buy Apple Care before the year - it was replaced 2 weeks before the year. We decided against the AC when we found out that it had 90 days on it.
You can still buy Apple Care for it before the year runs out if you want, that gets you another year.