Humor: The system has a day off

I came to work today - the terms are burning, I have to work overtime. I start the system that we develop - and I see Exception.

Began to understand. It turned out that in the system for statistics such an 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 be for several years.