Steffen is the tech lead for one team in a large online retailer. It's the kind of online store that sell items with large enough prices that financing is an important function. They offer loans through a payment provider, and the payment provider handles all of the details.

Anything relating to finance is complicated, and loans can get very complicated, especially these shorter-term loans. Generating a payment plan for a customer's order means creating a list of all the expected monthly installments, including the due dates, the remaining principal on the loan after the payment, how much goes to principal repayment, how much goes to interest, and the new balance of the loan after interest is added. It's a lot of information, and it's information that the payment provider can provide.

But, in the interests of keeping their application responsive when providing a potential payment plan to their end user, Steffen's company opted to implement their own financial calculations. For actual payment handling, they'd still let the payment provider handle it, but for showing the customer "if you order now, this is what your payment plan will look like!", they could do that all on their side.

Or could they? While it's complicated to calculate out all the loan elements, it's also well understood. There are libraries for it, there are copy/pasteable code snippets for it. It was complicated, but not hard.

After ten days of effort, however, it wasn't working. Their test plan was simple: since the payment provider was the source of truth, they generated 10,000 sample payment plans. They fed those plans into their system, then into the payment provider's system, and then compared the results. And the results mostly matched. Mostly.

There were a handful of test cases that were off by hundreds of Euros. That was way too much. Steffen called a meeting with the payment provider's technical staff, and developers from both companies sat down to go through the code, line by line.

It was a long, tedious meeting, and while questions were raised about the varying implementations, by the end of the meeting, both companies agreed: the code was correct. But the output was wrong. The meeting ended with nothing resolved.

So Steffen grabbed one of the developers, and they sat down to go through the test cases. They chose the one which had the largest deviation from the payment provider's values, and walked through each month of the payment plan.

On the first month, both Steffen's code and the payment provider were in perfect agreement. But on the second month, Steffen's code said it was €932.28, while the payment provider said that it was €631.54. That was more than €300 off. But on the third month, things got weirder: the payment provider's plan said that the amount of interest owed went up. Loans don't work that way: as you pay the principal, the amount of interest you pay each month goes down. "Do you think they got it wrong?" Steffen wondered.

It seemed promising, but then Steffen looked at the results in more detail. They had generated 10,000 random sample loans. This included random dates ranging from antiquity all the way out to the 30th century. It was very thorough testing.

This particular loan's first due date was October 1st, 1582. When Steffen spotted that, he laughed and turned to the developer. "This is a joke, right?" When the developer didn't see the humor, Steffen explained.

You see, Steffen didn't start life as a software developer. Before moving into the industry, he worked as a professional historian. And October 1582 was a very special month in history. A unique point in history: it was the month the Western world shifted from the Julian calendar to the Gregorian calendar.

The Julian calendar assumed a year was exactly 365.25 days, which is not correct. Following that rule adds an extra leap day roughly every century. The Gregorian calendar was more precise, and mostly accurate about such thing. But to shift calendars, the Greogrian calendar needed to be synced with the seasons. That meant removing a few centuries worth of extra days left over from the Julian calendar. Call it "paying down calendrical debt". So, October 4th, 1582 was followed by October 15th, 1582. Ten days just vanished, and October 1582 had only 21 days in it.

The payment provider's calendar system handled this perfectly well. Steffen's calendar system did not. That explained why they over-charged for October, and it also explained why it seemed like the interest payment went up- November 1582 was a normal, 30-day November.

Since they would never be originating loans in the 16th century, Steffen's team generated a new set of test data, all after the start of the 21st century. This solved the problem, and after fixing a few minor bugs, their implementation matched the payment provider's.

In the end, the Pope removed ten days from October 1582, and Steffen's team put them back, spending almost exactly that time on fixing this bug.

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