Trouble writing with AVAssetWriterInput

I have setup my `AVCaptureSession` with input from the camera and output to `AVCaptureVideoDataOutput`:

private func setupCaptureOutputs() {
    let videoDataOutput = AVCaptureVideoDataOutput()
    guard captureSession.canAddOutput(videoDataOutput) else { return } // captureSession is instance var
    captureSession.addOutput(videoDataOutput)
    videoDataOutput.alwaysDiscardsLateVideoFrames = false
    videoDataOutput.setSampleBufferDelegate(self, queue: captureQueue)
}

I have also created `AVAssetWriter` with `AVAssetWriterInput`:

assetWriter = try? AVAssetWriter(outputURL: videoPath, fileType: AVFileTypeQuickTimeMovie)

let assistant = AVOutputSettingsAssistant(preset: AVOutputSettingsPreset1280x720)
let outputSettings = assistant?.videoSettings
let formatHint = assistant?.sourceVideoFormat

let assetWriterInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: outputSettings, sourceFormatHint: formatHint)
assetWriterInput.expectsMediaDataInRealTime = true
assetWriterInput.mediaTimeScale = 30

I start writing and start a session with `kCMTimeZero` on user's button tap in simple `IBAction`:

    @IBAction func toggleCapture(_ sender: UIButton!) {
        guard let aw = self.assetWriter else { return }
        guard let awi = self.assetWriterInput else { return }
        sessionStarted = !sessionStarted

        if sessionStarted == false {
            awi.markAsFinished()
            aw.endSession(atSourceTime: self.latestTimestamp!)
            aw.finishWriting {
                print("done writing")
            }
        } else {
            startDate = Date()
            aw.startWriting()
            aw.startSession(atSourceTime: self.latestTimestamp!)
        }
    }

In output's delegate method I get the latestTimestamp and append the sample to writer's input:

func captureOutput(_ captureOutput: AVCaptureOutput!,
    didOutputSampleBuffer sampleBuffer: CMSampleBuffer!,
    from connection: AVCaptureConnection!) {
    self.latestTimestamp = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
    guard let awi = self.assetWriterInput else { return }
    guard sessionStarted == true else { return }
    if self.assetWriter?.status == .writing {
        awi.append(sampleBuffer)
    }
}

When I download the app's container and look at the video file it has length of few hours and only last few seconds display actual video. How would I go about manipulating `CMTime` instances to actual recorded video length?