Every once in a while the time gets out of whack on my MBP 2014 running Yosemite. Just right now I noticed it was roughly half an hour slower. I fixed it by going to the Settings->Time Zone, unlocked the changes and it synchronized the time properly.
Last night it was running on battery, I also ran Windows 7 in a Parallels VM. I closed the lid (I didn't power it down), went to bed, opened it this morning, I connected it to a power source, I surfed a little bit, went outside, came back and I noticed the time difference.
Any ideas? Could it be because it was running on battery, then I closed the lid without powering it down? The time difference was big. I am going to keep an eye on it from now on.