Why are my Mavericks Finder folders arranged by the wrong year?

I upgraded to Mavericks ages ago, but only just noticed this problem. Having arranged and sorted a folder by date modified, using icons, all items in the folder are listed as the wrong year. I have items grouped as being "Today", that's correct! But I then have "2013" items (which were all modified in 2014, I checked with Get Info) and "2012" items (which were all modified in 2013, again I checked them.

Why is this happening? How do I fix this?