Testing things that involve dates or times

Many applications rely on functions like datetime.datetime.now() or time.time() to create a timestamp. When we use one of these functions with a unit test, the results are essentially impossible to predict. This is an interesting dependency injection problem here: our application depends on a class that we would like to replace only when we're testing. The datetime package must be tested separately and a replacement used when testing our application.

One option is to design our application to avoid now() and utcnow(). Instead of using these methods directly, we can create a factory function that emits timestamps. For test purposes, this function can be replaced with one that produces known results. It seems awkward to avoid using the now() method in a complex application.

Another option is to avoid direct use of the datetime class entirely. This requires designing classes and modules that wrap the datetime class. A...