Okay, I've been thinking this out, and
there's really only four ways this could be happening.
1. I'm no longer connected with reality. Could be insanity, could be brain-in-a-vat, Cartesian demon, whatever. Really nothing I can do, if so – I guess I should avoid trying to go places, cook, etc., just in case. Other than that, I'll just dismiss this one.
2. It's a hoax. Preposterous, but it's more likely than any of these other things.
3. It's the 27th, and whatever erased all the memories of the last week and put everyone back in their places somehow missed all the computers. Impossible, but fits the observations perfectly – better than 2, certainly, and probably better than 1.
4. It's the 20th, and somehow my computer is seeing the future. There's the time-travel paradox is the main problem with this one – especially since the posts from the 20th aren't the ones I made. I suppose you could postulate an alternative universe, though, and that would also fit the observations perfectly.
Now, the question is: how do I tell which one it is?
2, I'll know pretty soon. A hoax this huge will fall apart fast, and make a big splash. Then we'll laugh at the clever little hackers, reset our clocks, and go back to life as before.
Wait. My watch says the 27th. This is inconsistent with it being hackers. We'll shelve 2 for the moment.
As for the difference between 3 and 4, all I can imagine is astronomy. If it's the 27th, the sun, moon, and stars should be in the wrong place. Moon is easiest – a week is a quarter of a lunar month. According to
this site, the moon would be waxing gibbous on the 27th, and therefore would be waxing crescent on the 20th.
...aw, man – I think I'll have to go outside for this one. Further reports after observations.