Comment by MadVikingGod

This is all possible and quite neat to dive into the specifics, but if you really want to be able swap a std lib call, just turn it into a variable and change it.

  // code.go
  var now = time.Now

  // code_test.go
  func TestCode(t *testing.T) {
      nowSwap := now
      t.Cleanup (func() {
          now = nowSwap
      }
      now = func() time.Time {
          return time.Date(...)
      }
  }

Examples Code: https://github.com/open-telemetry/opentelemetry-go/blob/main... Test: https://github.com/open-telemetry/opentelemetry-go/blob/490f...