Frozen uptime on iOS / iPhone

Question

Would someone know why I'm experiencing weird uptime with the following method?

NSProcessInfo *processInfo = [NSProcessInfo processInfo];
NSTimeInterval systemUptime = [processInfo systemUptime];

For first minutes, everything seems fine, but when I come back on the app hours or days laters the uptime is still the same : 30min, or 1h34... it seems to freeze at a random moment. Mostly on iPhone 4 (rarely on Simulator or iPad)

It could be linked to my way of showing it:

+ (NSTimeInterval)uptime:(NSNumber **)days hours:(NSNumber **)hours mins:(NSNumber **)mins
{
    NSProcessInfo *processInfo = [NSProcessInfo processInfo];
        //START UPTIME///////
    NSTimeInterval systemUptime = [processInfo systemUptime];
        // Get the system calendar
    NSCalendar *sysCalendar = [NSCalendar currentCalendar];
        // Create the NSDates
    NSDate *date = [[NSDate alloc] initWithTimeIntervalSinceNow:(0-systemUptime)]; 
    unsigned int unitFlags = NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit;
    NSDateComponents *c = [sysCalendar components:unitFlags fromDate:date toDate:[NSDate date]  options:0]; 
        //NSString *uptimeString = [NSString stringWithFormat:@"%dd %dh %dmin", [c day],[c hour],[c minute]];
    *days = [NSNumber numberWithInt:[c day]];
    *hours = [NSNumber numberWithInt:[c hour]];
    *mins = [NSNumber numberWithInt:[c minute]];
    [date release];
        //END UPTIME////////

    return systemUptime;
}

And later in the code:

NSNumber *uptimeDays, *uptimeHours, *uptimeMins;
[CJGDevice uptime:&uptimeDays hours:&uptimeHours mins:&uptimeMins];
NSString *uptimeString = [NSString stringWithFormat:@"%@d %@h %@min",
                          [uptimeDays stringValue],
                          [uptimeHours stringValue],
                          [uptimeMins stringValue]];

EDIT: after 3 days recording the results on iPad and iPhone I can see that this uptime is wrong, the time is running too slowly, the more we wait the more it's obvious that it's late

Answer 1

This method does the trick:

- (time_t)uptime
{
    struct timeval boottime;
    int mib[2] = {CTL_KERN, KERN_BOOTTIME};
    size_t size = sizeof(boottime);
    time_t now;
    time_t uptime = -1;

    (void)time(&now);    

    if (sysctl(mib, 2, &boottime, &size, NULL, 0) != -1 && boottime.tv_sec != 0) 
    {
        uptime = now - boottime.tv_sec;
    }
    return uptime;
}

Thanks to Alastair Stuart for the link.

Answer 2

That method is the amount of time the system has been awake since boot (not real-world clock time) and iOS devices typically spend a lot of time sleeping. That's why it's not increasing as much as you expect.

Source: http://developer.apple.com/library/ios/releasenotes/Cocoa/Foundation.html