if you realise that the creation date of all your clips are at a very similar time, perhaps the metadata was replace by the time you transcoded, downloaded or copied the footage. See the guide on transcoded media to read about alternate method to get the correct creation time.

If you got an obvious reference point, like the slate, you can get the offset you need by setting an in/out point at the slate marker and the point where the slate actually happens in the clip. Drop the clip with in/out point onto a timeline and you can get the offset. If you don't have a good reference point, just take your camera out and see how much different is it compared to your phone's clock.

But in my case, the offset was too large that the markers are in an entirely different clip then it should be. I had to look at the timestamp of the first slate marker that I have on the Markit! app, and subtract it with the time of the slate indicated by the clip.

The creation time of the clip is at 30 Jun 2018 5:09:32pm, and the slate's timecode is 17:09, so the slate is happening at 5:09:49pm according to the camera's clock. And in my app, it should be 5:15:05pm

Now google for a time calculator like this to find the time difference. Or you might not need one at all.

Now we have to put the json file and the original fcpxml file through the Markit! desktop app with an offset value. Remember to put a minus sign on any second or minute you have inputted if the camera clock is SLOWER than your phone. You can also key in the offset in seconds like 80 seconds instead of 1 minute and 20 seconds.

Now press the offset button, NOT the next button.

That makes so much more sense.

So that's it! See the finished film here.