NY wrote
Rod Speed wrote
Andrew wrote
Jim GM4DHJ ... wrote
Scott wrote

How does my car clock work? It cannot be quartz or radio controlled
as it runs slightly fast. It cannot be synchronous because the supply
is DC. Is there any form of adjustment to get it to run to time?

NEAR ENOUGH IS GOOD ENOUGH YOU HAVE TO RESET IT TWICE A YEAR ANYWAY

Having slightly fast is usually not an issue if you need to get
somewhere at a particular time.

Our schools have time of day based speed limits.

That can bite you with those.

Are the speed limits signposted

Yep.

(eg an electronic sign whose numbers change at different times of day),

Only very recently, in just the last couple of years. Solar powered.
But the fixed rather large signs have been there for decades now.
https://goo.gl/maps/ZHpZbMekZgd6Tc9BA

or does "everybody just know" what/when the speed limits are?

No, all of ours are always signposted.

Variable speed limits according to time of day outside a school is a very
good idea:

Yeah, it works very well and only applys for an
hour and a half at the start and end of the school
day, not for the whole of the school day.

it saves the UK convention of setting a speed limit which is absurdly low
for most of the day or night, except for the critical hours when schools
are coming or going.

Yeah, thats stupid. Some of our states, not mine,
have the speed limit applying even in school
holidays, but not my state.