The date bug that cost us a $120K contract and what i built to make sure it never happens again

I still remember where i was when the email came in. December 25th. Christmas morning. Phone in hand while having breakfast, and there is an email from our client's CTO. No greetings, Just "We're terminating the contract. Our legal team will be in touch" We lost a 120K a year contract. On a Christmas morning because of a date calculation bug that none of us, not a person on a team of 5 experienced developers had ever experienced. Here's what happened The file nobody wanted to touch Every enginee