Because OCT 31 = DEC 25.
Because they (some of them) don't have a garbage collector.
It's the only way they can get a string of dates.
Because OCT 31=DEC 25
Because Dec 25 is Oct 31
Squash one and deal with the other later.
Only once, and then you are subtracting it from 20.
Because Oct 31 = Dec 25