Question

I’m getting CMDeviceMotion objects from CMMotionManager. One of the properties of the CMDeviceMotion is timestamp, which is expressed as a NSTimeInterval (double). This allows for “sub millisecond” timestamp precision, according to documentation.

[motionManager startDeviceMotionUpdatesToQueue:motionQueue withHandler:^(CMDeviceMotion *motion, NSError *error) { 
  NSLog(@"Sample: %d Timestamp: %f ",counter,  motion.timestamp);
}

Unfortunately, NSTimeInterval is calculated since last device boot, posing significant challenges to using it in its raw form.

Does anyone have a working code to convert this NSTimeInterval into a Unix like timestamp (UTC timezone)?

Thank you!

Answer 1

I had a similar problem when comparing magnetometer values with CoreMotion events. If you want to transform these NSTimeIntervals you just need to calculate the offset once:

// during initialisation

// Get NSTimeInterval of uptime i.e. the delta: now - bootTime
NSTimeInterval uptime = [NSProcessInfo processInfo].systemUptime;

// Now since 1970
NSTimeInterval nowTimeIntervalSince1970 = [[NSDate date] timeIntervalSince1970];

// Voila our offset
self.offset = nowTimeIntervalSince1970 - uptime;