Get the precise time of system bootup on iOS/OS X

In OSX you could use sysctl(). This is how the OSX Unix utility uptime does it. Source code is available - search for boottime.

Fair warning though, in iOS i have no idea if this would work.

UPDATE: found some code :)

#include <sys/types.h>
#include <sys/sysctl.h>  

#define MIB_SIZE 2  

int mib[MIB_SIZE];
size_t size;
struct timeval  boottime;

mib[0] = CTL_KERN;
mib[1] = KERN_BOOTTIME;
size = sizeof(boottime);
if (sysctl(mib, MIB_SIZE, &boottime, &size, NULL, 0) != -1)
{
    // successful call
    NSDate* bootDate = [NSDate dateWithTimeIntervalSince1970:
                               boottime.tv_sec + boottime.tv_usec / 1.e6];
}

see if this works...

The accepted answer, using systcl, works, but the values returned by sysctl for KERN_BOOTTIME, at least in my testing (Darwin Kernel Version 11.4.2), are always in whole seconds (the microseconds field, tv_usec, is 0). This means the resulting time may be up to 1 second off, which is not very accurate.

Also, having compared that value, to one derived experimentally from the difference between the REALTIME_CLOCK and CALENDAR_CLOCK, they sometimes differ by a couple seconds, so its not clear whether the KERN_BOOTTIME value corresponds exactly to the time-basis for the uptime clocks.