I was about to comment on how this is because July and August were added in honor of Julius and Augustus Caesar, but it turns out they were renamed from Quintil

I was about to comment on how this is because July and August were added in honor of Julius and Augustus Caesar, but it turns out they were renamed from Quintilis and Sextilis (which would also be misnumbered). It was really January and February that were added later, causing the off-by-2 error :) Source: https://www.crowl.org/Lawrence/time/months.html https://www.crowl.org/Lawrence/time/months.html