I really like such kind of kata, good description, includes tests... but mostly because it looks easy when reading the description but than it develops into a real brain teaser when you're on it. And thanks for reminding me how much I hate date and time manipulation in JS ;-)

Your second example in the description is contradicting with your code example, if I understood it right the code example should be:

array_diff([1,2,2], [2]) // [1]