Getting iOS system uptime, that doesn't pause when asleep

I'm looking for a way to get an absolute, always-incrementing system uptime on iOS.

It should return the time since the device was last rebooted, and not be affected by changes to the system date.

All the methods I can find either pause when the device is asleep (CACurrentMediaTime, [NSProcessInfo systemUptime], mach_absolute_time) or are changed when the system date changes (sysctl/KERN_BOOTTIME).

Any ideas?