I will do my best to describe the bug… I can reproduce it using the comics posted during January of 2013. If you go the the very first post in January which should happen on the 2013/01/02 the post actually shows the comic. if you type in the dates in the address bar as: http://www.penny-arcade.com/comic/2013/01/01 you will get a comic, however; using the buttons to progress forward you will quickly start to skip comics and sometimes whole sets of days(you will skip forward to Wen or Friday). I should note that the Jan 2013 dates were ( Wen2 Fri4 Mon7 Wen9 etc) if you type in these dates in the address bar you will get closer to the correct comics however progressing forward using the buttons will cause the same symptoms to occur. I noticed the skip because I have been saving the comics day by day since 2000 and I was going back to download the high resolution versions for 2012-2013-and2014. My old archive and the new "comics" are not matching up. I could be wrong but if you watch the dates in the address bar you will notice comics posted on odd days Tue, Sat, etc. Hope that helps explain the problem. I would suspect that the problem lies in how database see the dates posted etc.
2013 03 08 is missing… but after 2013 03 11 the date jump subsides for some time and the buttons work again.
The bug reoccurs on 2013 11 01 - 2013 11 08 and on
2013 12 27 missing
2012 has the same problems up till mid March then everything works for a while then mid Nov broken again.