A test failed in my Medicare reimbursement engine on a day nobody touched the code. No commit, no deploy, no new data. I read the diff twice looking for the change that broke it, and there was no diff to read. What had changed was the date. The test had a hardcoded date in it, and the calendar had finally walked past it. The test was not wrong the day it was written. It was wrong now. That difference turned out to be the most useful thing I learned this year.
The Test That Expired
The engine handles remote therapeutic monitoring reimbursement for Medicare. Some records can be backdated, but only within a 72-hour window, and a test checked that logic with a fixture date somebody typed in by hand. The day it was written, that date sat safely inside the window, and the test passed for the right reasons. Then every day after that moved it closer to the edge. Nothing in the code changed. The fixture aged past the window the way milk ages past its label, quietly and on schedule, and one morning the test went red. A false alarm: a correct system, a failing check.
The embarrassing part is how long I stared at the code before I looked at the fixture. When a test fails, you assume the code drifted. It took me a while to accept the other possibility: that the test drifted while the code stood still.
