Unlike UTC that stays constant across the globe human time varies a lot. We long ago carved the globe up into 24 time zones so that - more or less - the sun will be overhead at noon in each zone.

Nothing difficult about that you might say. But then people start to get involved, and everything gets a whole lot messier. Neat straight line time-zone boundaries are shifted a bit to avoid slicing countries in half; the time itself is shifted to allow for lighter evenings in the summer (or lighter mornings in the winter if you like); regions are joined up or separated and we end up with a whole bunch of time-zones.

You really don't want to know the complications unless you have to: did you know that India is UTC+5h30 or that Arizona doesn't have Daylight Saving Time. Where I live in western France is on the standard European UTC+1 designed for countries far to our east so we get very light evenings!

UTC is what used to be known as Greenwich Mean Time (GMT). It stands for Universal Co-ordinated Time - yes, I know that would be UCT but the French and English speakers couldn't choose between UCT and TUC so we got UTC.

UTC is the international time standard and these days pretty much all other time is linked into UTC one way or another. Have you ever wondered how the clock in your computer keept time? Most likely it is getting time signals from the Internet. There are a whole bunch of specialised computers out there that do nothing but respond to requests for time signals. Usually Windows is set to do this for you automatically

UTC ticks over regularly and constantly, 24/7 365 days a year. It doesn't know anything about Daylight Saving, Summer Time or about time-zones. (Though actually it's the same as UK winter time.)