Well every equinox comes after a solstice, which came after an equinox, which came after a solstice... and that's just a year's worth. Good thing he doesn't live on Discworld with 8 seasons to keep track of!
The line was an internal thought, if i recall. The books represent his case files, written up at some unspecified point after each case was closed (much later in the case of Changes, granted). Billy refers to the case files in the DFRPG, and he has all the information in the books up to Small Favor*. So his mental state at the time it was happening shouldn't mess up his vocabulary years later. I mean, if he had permanent psychological our neurological damage at the time of the case file's writeup, he'd have those aphasic mistakes during the whole book.
* I wonder if the DFRPG will be completed as an in-character book in the Dresdenverse, given what happened to Kirby not too long after the "draft" that we see as the RPG-in-progress?