How to turn CoreMotion timestamp into proper Date? - ios

Question

I'm looking for a way to turn timestamp found on CoreMotion events into proper, high precision "wall date-time". I want to correlate events on iPhone and Apple Watch, so need to translate those timestamps to the single domain.
I tested several approaches on internet, like this: https://stackoverflow.com/a/53250802. Which gives utterly wrong results.
And this
extension TimeInterval {
static var bootTime = Date().timeIntervalSince1970 - ProcessInfo.processInfo.systemUptime
var unix: TimeInterval {
return (TimeInterval. bootTime + self)
}
}
which seem to work on first sight. My tests shown that over time, there is increasing discrepancy between result unix TimeInterval:
In this picture you can see sessions that gather events. Single session events has the same color. On upper line you see watch events and on lower phone events. We see that it seems like timestamp on watchOS advances slower then on iPhone. This looks like timestamp might be connected to processor cycle count?
My question is: How to translate CoreMotion timestamps into Date in proper fashion both on iPhone and watchOS?

Answer 1

This is actually correct solution.
extension TimeInterval {
static var bootTime = Date().timeIntervalSince1970 - ProcessInfo.processInfo.systemUptime
var unix: TimeInterval {
return (TimeInterval. bootTime + self)
}
}
My problem with lagged timestamps on watchOS was connected to cluttering processing queue, so event handlers was lagged over time more and more. For some reason, MotionManager does not react to stopMotionDeviceUpdates() and I was getting those over and over again, despited the fact, I turned it off. I just tried to process too much data - I increased sensor frequency to 1/100.