CSLogItem时间戳格式(NSTimeInterval从什么时候开始?)

I'm logging CMAccelerometerData on the iphone. CMAcceleration is a subclass of CMLogItem, which defines a timestamp. The timestamp is very important to me, as I have to match CMAcceleration data to positions obtain via GPS. GPS gives me a NSDate object, which is good; but CMLogItem gives me a number that I cant relate to anything.I'm logging CMAccelerometerData on the iphone.