Days in a year ok (except leap years). But seconds in a day are round (discounting days with leap seconds). 24 * 60 * 60 = 86400, which is divisible by two. Did you mean they are not based on the decimal system? I’d be up for a decimal based time system and a reorganised calendar, but that wasn’t the topic of discussion here.

Oops, I thought seconds were defined by the meter at some point. Nope, a pendulum 1/40000 of the distance from the pole to the equator just happens to measure the second near-perfectly, but the second stayed defined by astronomical motions until the atomic standard. Still, do to said variability of the Earth's rotation since then it's 86400.002, so even if it stopped changing we'd need leap seconds.

The point being that even if you get rid of timezones the calendar will still suck to work with. I question whether we should even have fixed days, months and years, if the time doesn't relate to the position of the sun in the sky. You might as well just go with Unix epoch, and leave days to be informal. Of course, then you'd have to calculate multiples of 86400 a lot to set appointments. Maybe we need a new decimal second as well.