Maybe subtracting two per day, or the "leave by five" had an implied "at the very latest." Surely he did the math before putting the number in the title of his blog post.
He worked four 10 hour days and called it 32 hours a week. We don’t have any reason to trust his math.
There is a reason why it's called a 9-5 job, and a 40-hour work week. They are the 'standard.'