900*.8         == 720
(300*.8) * 3   == 720 
~~(300*.8) * 3 == 720

Edit : ok, things start to differ after 4th year ( ~~(900*.8*.8*.8*.8) != ~~((((300*.8)*.8)*.8*.8)*3 )

Ok do not know why assertSimilar don't work... Anyway I sort the object to be safe.

I already use assertSimilar(), I don't get it... Any idea ?
I will find a fix soon.

Interesting strategy! Brilliant!

Possibly you could also compare the regex's test method to an untouched regex's test method?

multipleof3Regex.test === /abc/.test

Done. The first option only; actually I'm checking the object itself, as people might not modify the prototype's method but the object's one. Thanks for pointing it out.

Done!