Here's what has happened to me in both the US and UK systems.
In America, you call your doctor and request an appointment when it's convenient for you.
There was no choice over appointment times — the assumption is that if you're ill, you're going to come to the doctor when they say. In America, In Britain, I better show up when I'm told.
Nonetheless, Trump's tweet raises an interesting question: Is the British system of universal free care, via a single-payer "socialized" system, better or worse than the healthcare you get in the US?
Until 2014, I spent half my life in the US and half of it in the UK, so I'm used to both countries' healthcare systems.
The context here is that the NHS regularly releases its most recent stats on accident and emergency room waiting times.
The headline number is that 85% of patients are seen within four hours.