Also to make sure that the calendar date changes at a time when most people are asleep or otherwise don't care about it.

Without timezones, everybody who isn't lucky enough to live near the new meridian will have the calendar date change at some point in the middle of their waking hours, which is a great recipe for confusion.

It'd also mean that in those places, anything defined on a day level (like holidays) would either have to start and end right in the middle of the solar day, or you'd have to start specifying starting and ending hours, too.

And if a place then starts to define standard starting/ending hours which approximate the former local midnight, you basically end up reintroducing time zones through the back door.

I really want daylight savings to die, and time zones, yea why not.. it'd take a bit of adjustment, but when you know that CountryX is N units ahead of, or behind your own, you will know what time of day it is there simply by subtracting or adding the difference and compare with your local brightness which you know well, assuming you're on somewhat similar lattitudes.

Let's also do decimal time while we're at it, because nobody cares for base60 anyway.

My argument for NOT changing anything is the amount of assumptions that have been built into all sorts of critical and non-critical software, it'll simply be too expensive at this point, not to say dangerous.