Timezone Demystification
This is just a vulgarization to explain the different “time” concept we could encounter. It’s suppose to ease the understanding and might not be scientific perfect.
Check resources for detailed definition.
UTC
Universal Time Coordinated is one recent technical time scale where the earth is divided into 24 UTC Zones. Each zone is referenced by an offset, positive or negative, relative to the UTC time. It’s purely a geographical time where the delimitation is based on the longitude.
How about GMT
GMT used to be the previous standards for defining time scale. As its definition is unclear, it’s deprecated to use GMT.
Regional Timezone
Because of our social aspect, we need to have a way to refer to a point in time to synchronize with others people for almost anything.
We also unfortunately didn’t organize our territories to match the UTC Stripes which means that some countries are not “aligned” with their Geographical time.
From this, some "standard clock" has been put in place , like CET, PST etc.. , so entities can aligned all together so that they share the same "regional timezone".
City Timezone
At the end, it's the authoritative local administration, that ultimately decide in which timezone the area of influence of this administration will use as civil time. So you can represent City Time as a pointer to an official UTC timezone that might change either periodically or occasionally.
What is important to keep in mind, is that timezone doesn't move. UTC+X still at this offset, CET still UTC+1 all the time.
E.g Samoa in 2011, Apia Time, change from UTC-11 to UTC+13 to align themself with neighbour countries timeline.
The timezone still the same, it's just the pointer of Apia Time that changes.
Daylight Saving
To add human complexity, the concept of Daylight Saving was introduced to offset our lifestyle based on the year period to align more our day life with the sun time.
Why not Unique Timezone ?
This is directly linked to the explanation to Regional and City Timezone.
Even if there is cultural difference, and changes during the year, all around the world we have roughly the same “day representation” :
Between 6 to 12 its Morning, usual stores open, we all wake. Up and have breakfast
12 to 14 is Noon, where we have lunch break.
14 to 21 is Afternoon, finished by a dinner.
21 to 6 is Nighttime where stores closes and we sleep.
Again, it’s a very subjective view, but if you move to another country far away from your home, you will use theses times as reference as you were on your home place.
Having a unique timezone will break this mental representation and be very ineffective compare to the use cases it could solve.
First, unique timezone mean defining a “0”. UTC is extremly Eurocentric, there is nothing technically stoping to put the "0" to the middle of Pacific Ocean.
Even if we use UTC, as European it will be ok to adapt, but as US or Asian, you will have to learn which time all the “’habits” are.
As we usually have more relation with our closer neighbours and we don’t travel far that frequently, it’s then a least worst decision to have multiple timezone.