The clock a test sets. set/1 overrides the configured clock for the rest of the current process and answers the moment it set. So a test that needs a fixed time names it once, and every call the port makes reads it.

The configured clock is a zero-arity function, so a test needs no mock and no behaviour of its own. set/1 installs a closure over the moment. Mediate.Config.resolve/0 finds it from the test process and from any process in its $callers chain.