I was just testing with TIO, it looks like the version of `cal` on there is different to the common one. The ⎕SE.UCMD'Calendar 2021' version should work cross p

I was just testing with TIO, it looks like the version of `cal` on there is different to the common one. The ⎕SE.UCMD'Calendar 2021' version should work cross platform (as it doesn't rely on anything external).