In OTL, time zones were introduced in the late 19th century when time tables for long-distance trains, for instance, began to require more precision and standardization across wider areas than a sundial could provide. The day had been divided into 24 equal hours long before that. 

In an ATL, how would it have affected the development of mechanical and electronic clocks and watches – also time-keeping as a whole – if social custom required that local sun time with hours of flexible length was to be respected in the following way? How would such a society differ from ours?

* Each calendar day is separated by local sunrise and sunset into two phases, *light* and *night*.
* Each phase is divided into 12 hours – just for familiarity – of equal length (in a phase)! Therefore, the length of hours varies from day to day; *light* hours are longer than *night* hours in summer, but shorter in winter.
* Sunset starts the calendar day and *light* at 00:00, *night* starts at 12:00. Consequently, there’s no need for daylight saving time – the very idea would be absurd.

A stop watch (absolute *chronometer*) and a clock (relative *chronograph*) would have to use different units of time from each other. 

“Local time” may acquire a looser meaning over time, i.e. not the observed sunset and sunrise at the location of the clock would be decisive, but the nominal times from some kind of capital would be applied to a wider area. 

With sundials, the length of hours may even have varied throughout a day, but let’s assume the difference is so small that people tried to implement hours constant per day when they started building mechanical or electronic clocks.

PS: Note that in OTL, (atomic) clocks are now so accurate that we occasionally need leap seconds to synchronize relative with absolute time, because a second is not exactly 1/86400th of any day.