Batman Adventures: The Lost Years Vol 1 2 was published on January 2, 1998. The nearest Wednesday is December 31, 1997, but for some reason it's been automatically categorized in "December 0, 1997 (Publication)" as well as "1998, January (Publication)".
Did I do something wrong or is there a problem with the template?
Just in case anyone's curious, I'll explain what was wrong (so I feel validated for having fixed it).
Because the number of days until the nearest Wednesday would be a negative number in the case here, the template knew to go back to the previous month. However, the template treated January(1) minus 1 as 0 (not 12; December). The template then relied on a second template to provide the number of days in the month based on the number value of that month, and I hadn't accounted for the fact that there would be some calculations where the month value would turn out to be 0 or 13. Fixed that, and that fixed the main Day Categorization template.