This is not specific to the Galaxy S6, but it happens to be the phone I am using right now and it's happened twice today - and also to my daughter in law with a Verizon phone (but the Note 4). The time jumps back an hour.
Investigation revealed that the Auto Time Zone was resetting itself to Alaska Time - both u of us could turn off the Auto time zone, manually set it to Pacific Time, and all was well. Reset to Auto Time zone, and ait jumped to Alaska Time again. Leave it off, manually set to Pacific, it's all good.
Earlier today when it happened, I didn't change settings, but did reboot the phone and it came up the correct time. But a couple of hours later, it was off by an hour again. That's when we discovered the time zone was showing as Alaska. Just an FYI, I'm not even close to a border, time one or otherwise, I'm in Northern CA between San Francisco and Sacramento with excellent 4G Coverage.
I've seen several questions recently about time zones being "off", by one or more hours. What causes it and how do we make it stop? It's confusing when you glance at the phone and have to double check with other clocks when it doesn't seem right!