It's actually not the case that when it's 24 past the hour in one
country, it is in all countries.
There are a surprising number of places in the world with
idiosyncratic time zones that are offset from the major time zones,
generally by 15 minutes or 30 minutes.
The reason for the variability is that each jurisdication around the
world gets to make its own choice about how to set the local time.
This includes choosing whether to rely (or not) on the global time
zone system of 24 hourly segments, whether to adopt (or not) daylight
savings, and whether to divide (or not) large expanses into multiple
This results in numerous oddities.
For instance, there are many more than 24 time zones in the world...by
some counts as many as 39, plus a few areas without an officialy
Crossing from some zones to an adjacent zone can result in a time
difference as large as three and a half hours!
There are places in the world where more than two time zones meet at one spot.
And there are huge countries -- China, for instance, -- with only a
single time zone.
Given the fact that countries and other jurisdictions get to choose
how, when and why to set their clocks as they do, there isn't a single
point in time when the world's clocks suddenly became synchronized.
Nor is there even what one could call a smooth historical transition.
There is a very good overview and historical summary of the global
time zone system (and its numerous exceptions) at the Wikipedia online
The site includes a nice map of the world's time zones:
and if you want a good look at it, you should download the beautiful,
high-resolution verion here:
I think this should give you a terrific overview, and should provide
all the information you're after.
However, if there's anything else you need, just let me know by
posting a Request for Clarification, and I'm at your service.
search strategy -- Google serch on [ time zones ]