I don't remember all details. My co-workers searched to understand the bug and he discovered that changing the creation date for just some second made the bug disappear. I then discovered he called the function with a bad argument. Strangely using an integer instead of a time object (I am not sure about that but you get the idea) will render the exact same result most of the time. I corrected the bug by using the right function call. I then tried a brute force search to discover for how many time the bug occurs. And if remember well there were two close period of a few second each during wich the function bad called render a different result. It was something like 17 second and 3 minutes later another 10 second. I believe I simply tried for the current year. I might have tweeted about it some years ago.
It occurred only 27 second by year! Just to think about the probability to detect it knowing the code is used only about twice a week.