What are Time Zones and how can you use them?

Pt Mt Ct Et Us Time Zones


The world is split hierachically into time zones.


The splitting may be done over two dimensions:

  • longitudinal
  • or city (Island)


There are 24 longitudinal time zones (one time zone for one hour) that have an offset or acronym identifier

Utc Time Zone

City or Island

The world is split into city or Island time zones that have an id_identifier that take into account political divisions in order to apply seasonal adjustments known as Daylight Savings Time).

Note that the time zones are not split into countries because they would be too dependent in time on political and boundary changes.

The city time zone Europe/London is:

  • in standard time: UTC+0
  • in daylight time known as British Summer Time (BST): UTC+1

If you care about the local time, you should store the UTC time and the city time zone.


An identifier can be:

Full name

The Time Zone full name is an Id.

A full name timezone ID is the identifier for a City or Island timezone name.

They are more than an offset as they may be influenced by political decisions, not just earth geometry.

They define:

It's therefore the only way to define the beginning and the end of a day.

This CSV has the list of all time zones with their name, abbreviation, and DST settings.

They are names 1):

  • in the form Area/Location, e.g. Europe/London
  • in English


  • Area is the name of a big region such as a continent, an ocean
  • Location is the name of a specific location within the area (usually a city or small island)
  • Area and Location names have a maximum length of 14 characters

Longitudinal time zone


An abbreviation name or acronym represents a longitudinal time zone.

Time zone abbreviations like 'EST' are used by:

  • traditionally by human
  • and POSIX

The acronyms end up generally with:

Note that this is not always true. For instance, BST means British Summer Time and is a Daylight saving time.

For instance:

Pt Mt Ct Et Us Time Zones 2)

There is therefore no one-to-one relationship with the timezone id. Example: the America/Los_Angeles timezone id is:

  • in the PST timezone
  • and PDT timezone


A time zone identified by an offset is a longitudinal time zone (and not a political/civil time zone that may apply day light saving)

It's generally the gmt / UTC offset

Given a time zone ID, you can determine a UTC offset but the opposite is not true. There is not a 1:1 mapping from UTC offset back to a time zone id.

For example, the time zone for Sydney Australia/Sydney (New South Wales) is:

  • UTC+10 EST (Eastern standard time),
  • or UTC+11 EDT (Eastern Daylight time) during daylight savings time.


The timezone database 3) is also known as:

  • tz database,
  • tzdata,
  • zoneinfo database
  • IANA time zone database,
  • Olson database (in reference to the founder, Arthur David Olson)

This CSV has the list of all time zones with their name, abbreviation, and DST settings.

No timezone: Epoch / SQL Timestamp

epoch and the SQL timestamp does not store any time zone information.

Computer Language

timeZoneDesc = Intl.DateTimeFormat().resolvedOptions().timeZone;
console.log("Your time zone is: "+timeZoneDesc);
offsetMinutes = (new Date()).getTimezoneOffset();
let now = new Date();
console.log("UTC time (London) is: "+now.toISOString());
var timeZone = new Date(now.getTime()-1000*60*offsetMinutes);
console.log("Your local time ("+timeZoneDesc+") is: "+now.toLocaleString(window.navigator.language));
if (offsetMinutes <0) {
  op = "subtract";
} else {
  op = "add";
console.log("To go to UTC, you need to "+op+" "+Math.abs(offsetMinutes)+" minutes (ie "+offsetMinutes/60+" hour) from your local time. If you are in a time zone that has Daylight Savings Time, your zone may be off of 1 from UTC"); 


See http://www.worldtimezone.com/


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