I've written a function to return the time_t value corresponding to midnight on a given day. When there is no midnight for a given day, it returns the earliest time available; that situation can occur, for example, when Egypt enters daylight-saving time. This year, the time change takes effect at midnight on the night of April 29, so the clock goes directly from 23:59 to 01:00.

Now I'm writing unit tests for this function, and one of the tests should replicate the Egypt scenario. In Unix, I can accomplish it like this:

putenv("TZ", "Egypt", true);
tzset();

After doing that, further calls to localtime behave as if they're in Egypt instead of Minnesota, and my tests pass. Merely setting the environment variable doesn't have any effect on Windows, though. What can I do to make the unit test think it's somewhere else without affecting the rest of the programs running on the system?