dateByAddingTimeInterval method availability

I use dateByAddingTimeInterval method. I checked this method on iPhone with iOS 3.1开发者_Go百科 and it's worked! But in documentation said that this method available in iOS 4.0. Why this method worked in iOS 3.1?

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It looks like an error in the documentation. The header specifies:

- (id)dateByAddingTimeInterval:(NSTimeInterval)ti NS_AVAILABLE(10_5, 2_0);

So it's available in 10.5, and iOS 2.0. That's why it is present in iOS 3 and later.