// 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(...)
      }
  }

The way it works is that at the start of every function it adds an if statement that atomically checks whether or not the function has been intercepted, and if it did, then executes the replacement function instead.

My tool no longer works since it was rewriting GOPATH, and Go since effectively switched to Go Modules, but if you're persistent enough you can make it work with Go modules too — all you need to do is rewrite the Go module cache instead of GOPATH and you're good to go.