I experienced this myself and it was completely confusing!<p>Back in 2008 before I had a smartphone with GPS, I had a camera and a separate GPS device I would use to &quot;tag&quot; places where I took interesting pictures during a 6-month trip around the world.<p>When I got back home I ran some code to match picture timestamps with GPS timestamps, correcting for time zone offsets (GPS was GMT, pictures were local). All of the pictures of the rest of the countries matched just fine, except for China :) Since I had relatively few data points, it took me a while to realise there was a systematic error in the data, not something I was doing wrong with the timestamps.