The language isn’t loose by any means. In the case of the Date constructor, it specifies a minimum requirement. Nobody can stop implementors from doing more than that, though.
As to the “playgrounds”: Those are (in my opinion) useless. Just start your browser and open its developer tools. In their “Console”, you can fiddle around with JavaScript as much as you want. And that’s the original, not some kind of plugged-in “playground”. It’s also fast and free. You can also use Apple’s osascript command-line tool:
osascript -l JavaScript file.js
runs the code in file.js.