Humor: The system has a day off

I came to work today - the deadlines are on, I have to work overtime. I start the system that we are developing - and I see Exception.

They started to understand. It turned out that in the system for statistics, this expression is calculated
X = (number of days when the system was used) / (number of working days from the beginning of the month).

So if the beginning of the month falls on a day off - the system does not work that day! And this error seems to have been several years old.