Cmlogitem Timestamp: Why So Complicated

CMLogItem timestamp: Why so complicated?

Proper answer is:

extension CMLogItem {
    static let bootTime = Date(timeIntervalSinceNow: -ProcessInfo.processInfo.systemUptime)

    func startTime() -> Date {
        return CMLogItem.bootTime.addingTimeInterval(self.timestamp)
    }
}

This gives us stable, monotonic results, which is not a case, when bootTime is computed every time startTime is called.

iPhone - core motion timestamp

Seems to be fine the way you do it. But why do you create a new instance of CMMotionManager? I use:

if ([motionManager isDeviceMotionAvailable] && [motionManager isDeviceMotionActive]) {
    [motionManager stopDeviceMotionUpdates];
}

and to continue:

[motionManager startDeviceMotionUpdatesToQueue:operationQueue withHandler:deviceMotionHandler];

I was curious to see if there is still something wrong in iOS with the timestamps and thus I just tried out your code or similar thing using a static. Everything works as expected. Maybe your timestampReference is overwritten somewhere else?

How to get exact time since iOS device booted?

After investigation of the data you provide. It is possible to conclude that CMLog​Item:

The CMLog​Item class is a base class for Core Motion classes that
handle specific types of motion events. Objects of this class
represent a piece of time-tagged data that can be logged to a file.

CMLog​Item defines only a read-only timestamp property that records the time a motion-event measurement was taken. Not the exact time since device was booted.

By ProcessInfo.processInfo.systemUptime you going to get time from the last time it was restarted.

I believe data that you would like to obtain is quite specific and is under Private API of the iOS.

Nslog timestamp

Edit: Please see Nicolas Lauquin's answer. Per the comments, the following solution is not correct but is retained here for history (and because I can't delete it since it is currently marked accepted).

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The timestamp property is an NSTimeInterval, so you should be able to do:

NSLog(@"Motion at time: %@",
[NSDate dateWithTimeIntervalSinceReferenceDate:devicemotion.timestamp]);

NSTimeInterval is just a typedef'd double type, so you could use %f instead of %@ and log it directly.

Also, the docs don't indicate whether this timestamp is set against Apple's reference date or the standard *nix date, so you may need to use [NSDate dateWithTimeIntervalSince1970:] if the aforementioned method returns dates far in the future.

As @davidbitton has suggested the CMDeviceMotion's timestamp is relative to the last device boot, the correct NSDate could be derived by

NSDate *startupTime = [NSDate dateWithTimeIntervalSinceNow:
                          -1 * [[NSProcessInfo processInfo] systemUptime]];

NSDate *deviceMotionDate = [NSDate dateWithTimeInterval:devicemotion.timestamp 
                                              sinceDate:startupTime];

This should yield a roughly accurate NSDate object, assuming @davidbitton is correct. (reference: NSProcessInfo -systemUptime)

However, given how complicated this is, I would now suggest for simplicity that, given the nature of the timestamp property, that you log it in a format string as something like

"... event logged at %0.2f seconds since startup...", devicemotion.timestamp

In iOS, using Swift, how do you get the time of iPhone boot?

From the comments, I was able to find ProcessInfo.processInfo.systemUptime which is the time since last boot. From this, and Date(), I can get the time of start via:

time_of_last_boot = Date() - ProcessInfo.processInfo.systemUptime

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