Automatic time on Nexus 5X says it's 2017 already, one day ahead
I was just reading this article:

http://android.wonderhowto.com/how-to/set-your-android-atomic-time-for-perfectly-synced-clock-0170500/

It suggests your phone is using the NITZ service to sync time, which relies on good carrier service.

The article says that if this service is bad, it might take your phone out of properly synced time.

You can try installing the suggested app.

Without root, you won't be able to sync automatically, but you can sync manually every week or so.