Off by one errors is the bane of math, logic and programming. Did you for instance know that the earth rotates about 366 times around its own axis in a year? The rotation of the earth around the sun accounts for one apparent rotation being deducted, giving you 365 days.
One of the most hilarious and widespread cases of humans failing to understand the fence-post problem is that if you count out loud from 1 to 10 in 1s intervals, that's actually only 9 seconds. If you want 10 seconds you need to start from 0 or go to 11.