What surprises me (and makes me most skeptical) is that they can remember days by date. Did these people actually know what the date was every year of their lives? This implies to me that it is not just a camcorder-like memory that these people possess, but an efficient, numerical search ability on those memories. It makes me wonder how much one could duplicate this ability by purposefully associating numbers with days in a hierarchical structure (analogous to building an explicit tree structure on them).

Also fascinating to me is how this links with OCD-like behavior. My memory has always been considered very good (though not like these people), and I had extremely mild OCD during my preteen years, which changed its form when I became a teenager until I stopped manifesting most symptoms.

I also wonder what would happen if these people were asked to gather days by an arbitrary filter. For example, on which days during 1999 did it rain? Would they have to step through day-by-day and check each day, or would they be able to instantly run through the rainy days? The latter seems almost too powerful.