I did a bit of testing of my own, and I believe that the sequence “sky_presunset1_0” to “sky_presunset1_9” is not by month, but rather by day. Moreover, it does not increment the number by one every day. Most of the time it seems to jump forward by two numbers each day, but then mixes up the sequence in a way that I haven't yet figured out. The point of this, obviously, is to generate a completely different looking sky every day, and swap them around in a semi-random manner. What's interesting is how many of the commercial textures overlook this behavior.