Would this mean that it's travelled forward two days and so the last post (with a now-corrected date) has travelled back from the future? Or alternatively could the laptop have travelled back two days and so be submitting posts that were made in the future?
Is there actually a difference between the two?
On a less time-travelly note, I wonder what failure mode could cause this. I think the RTC battery (a lithium button cell) is flat or almost so as the real-time clock runs very slowly if at all when the laptop is unplugged and the main battery removed. I've also noticed that the main battery discharges quite significantly when the laptop is supposedly powered down. This all suggests that the RTC is mainly running using the main battery.
I doubt it's a time-sync error unless someone in pool.ntp.org is playing games with their NTP server :)
Edit: Windows refuses to synchronise over NTP unless the date matches making a time-sync error very unlikely.