CSLogItem timestamp format (NSTimeInterval since when?)

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.

It's not UNIX ti开发者_运维百科me for sure. This morning (17/11/2010 at 11H17am, in Australia), it was at 4090 seconds.

Any ideas of what that could be?

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Nevermind I found out, CMLogItem timestamp is a NSTimeInterval starting when the phone last boot up.