So I decided to play today, since I haven't played since the Sunday after launch and I've been dying to play. I saved in my house after I slept for 12 hours. It was close to 9PM when I slept, the day changed properly at midnight, then changed again at 3AM. I advanced 2 days in the space of 12 hours.

Then when I saved and quit playing, I slept again for 12 hours and the day didn't change until 3AM, but it didn't advance two days this time.

I saw someone post in the PC forum about the calendar/day de-syncing and re-syncing quite often, looks like another bug for Bethesda to solve.