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.