Nice solution! Is there a reason for the return before dirReduc(arr)? Just curious because I tried it without the return and it also worked.

I am wondering if this could be just sorted.slice(-1) without the [0]?

I think there is a typo, it should be: The round yellow light on top blinks to denote odd- (when lit) or even-numbered (when unlit) seconds (as written here: https://en.wikipedia.org/wiki/Mengenlehreuhr)
Please can you change it in the question?
Many thanks

Sorry, a bit confused, could someone explain how this code works - I don't understand how pop() works on an empty array? What exactly is being popped and pushed?

Nice solution, I find it very clear.
Can I ask why you multiply by 60000 to get the milliseconds? Why is milliseconds needed?

I'm having the exact same problem as Buddha_B
It works on chrome console but not here?

Basic tests
Test Passed: Value deep equals [3, 3, 3, 5, 5, 7, 7, 2, 9]
Expected: [1, 1, 1, 0, 0, 6, 6, 8, 8, 2, 3, 5, 9], instead got: [1, 1, 1, 0, 0, 6, 6, 8, 8, 9, 5, 2, 3]
Test Passed: Value deep equals [9, 9, 9, 9, 4, 4, 5, 5, 6, 6]
Test Passed: Value deep equals [1, 1, 2, 2, 3, 3, 4, 4, 5, 8]
Test Passed: Value deep equals [0, 0, 4, 4, 9, 9, 3, 5, 7, 8]

As you can see, in the tests that I pass, there is the same logic applied as the one that I do not pass. Not sure where I am going wrong?

Is the starting accumulator a 2 element array because the result needs to be a 2 element array? ie. does the format of the accumulator have the be the format of the final output?

Sorry if my question is basic but what is the line input = input.replace for?
Why does the input need to be replaced?