This article presents a grossly simplified version of events. The truth is that only some catholic countries switched in 1582. Britain and its colonies switched in 1752. Prussia switched in 1610. And so on.
Hadn't Britain established the C of E permanently by 1570, so it wasn't part of the Catholic world any more?
That might be. I am not a history experts. I just wanted to counter the notion that that one October 1582 was the singular special month where the switch happened. That was not the case and calendars stayed messy until the 20th century because of this.