Is there a clock in iOS that can be used that cannot be changed by the user
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I am trying to design a system where real-time events happen and I want to synchronise them to a clock.  [NSDate date] would normally be ok but the user could change this and cheat the system.  All I need is a clock that I can take relative times from (a 64-bit counter for example) - I don't need absolute time (or time of day etc).

Is there such an API I can use?

EDIT:  I would also like to add that this clock needs to be persistent over sessions of the application.