What unit of time does timeIntervalSinceDate return?

Well the NSDate docs show the function prototype as:

- (NSTimeInterval)timeIntervalSinceDate:(NSDate *)anotherDate

And the Foundation Data Types doc states:

NSTimeInterval is always specified in seconds; it yields sub-millisecond precision over a range of 10,000 years.

It returns the number of seconds, as an NSTimeInterval value.

From this documentation page:

NSTimeInterval is always specified in seconds; it yields sub-millisecond precision over a range of 10,000 years.