If the day started at 1:00 then by the second hour you would be at 2:00, even though only 1 hour has passed.
When the second day of the month starts, the day of the month is 2, even though only 1 day has passed.
I mean, numerically it does make sense to start at zero but it doesn’t seem to correspond to the way people think and talk.
Killing one of your friends might not be the optimal solution.
But one of your friends might think it is.