About time - how to unit test code that depends on time

Five approaches for making time-dependent C++ code unit-testable are compared. The naive approach of calling std::chrono::now() directly is untestable. Alternatives include alias clocks (requires separate builds), template specialization tricks (avoids separate builds), clock factories (singleton problems), injecting clocks via constructors, and passing timestamps as parameters. The recommended approach is passing timestamps directly to functions, which simplifies tests and avoids singletons, with template specialization as a fallback when precision loss is unacceptable.