For example for a puzzle i just solved I get:
Recent activity: Sep 0, 2014
For another puzzle i get:
Recent activity: Sep 2, 2014
but I havent joined this site until a few days ago.
Also regarding the ranking graph on the profile I think it should only start at the join date of the user. This way it currenty works it looks like I have been part of the site for a few years but never solved anything 
Just ran into this bug as well. For the puzzle that I just solved, the date is listed as ‘Nov 0, 2014’, while for the ones I solved yesterday it’s listed as ‘Nov 6, 2014’. Today is only November 2 though, and I was not aware I can travel through time
It seems the date is done with the day of week instead of the day of month. So 0 actually means Sunday, 1 for Monday and so on. 
That was exactly the case 
. We already fixed it, and the fix should be applied on the website soon.