Date/time logic is hard. Throw in time zones along with daylight saving and it's even harder. Recently a suite of tests that had happily been running for months started failing. There were no code changes and all the tests were somehow related to date/time ranges.
Despite this the production code was functioning as expected. It turns out the API was explicitly setting the locale to use
en-GB. However the suite of tests were not.
The fix was simple. Prior to the test fixtures executing, explicitly set the locale. In order to test this assumption and see the tests pass, a temporary change on the development machine was required.
The locale was set on the development machine to another region. In this case setting to
en-US was enough to cause the tests to fail. After the code change the tests passed. Any locale can be used as long as the date format differs.
This idea is pretty easy, and is very close to my technique of pulling the plug on automated tests. The test suite can now be run on any machine, even those incorrectly configured and we can be sure the tests will still pass.
Going forward for any date/time tests I will make an active decision to temporarily change my regional settings. With more codebases utilizing the cloud, relying on implicit configuration should be avoided where possible. In fact I would bet a large sum of money that many codebases out there would fail this temporary locale change. Give it a go - pull the plug.