Marshmallow Time Drift bug
Android 6.0.1 apparently fixes the issue. I'm running it on my Nexus, so I can't say about custom ROMs, but it should be fixed in 6.0.1 based ROMs.

The bug was detected by me in 6.0 first because my alarm rang 21 minutes late. It was about a week after I bought the phone. A reboot fixed the time, and since I've upgraded to 6.0.1, I haven't seen an issue with time.