Zone offsetĪ zone offset is an amount that is added to or subtracted from UTC based on the location of the event around the world relative to the prime meridian. Time zone rules have to take into account zone offsets plus any daylight savings modifications to wall time that apply. Time zoneĪ time zone is a set of rules for determining the local time (wall time) as it relates to incremental time (as used in most computing systems) for a particular geographical region, and vice versa. Other examples of floating time events include the publication date for an issue of a newspaper, the date the Rio Olympics starts, the time the New Year starts, office hours set to '9 to 5' regardless of time zone, and so on. When you apply time zone information to floating times they produce a range of acceptable incremental time values.įloating times are not attached and should never be attached to a particular time zone. We refer to these sorts of time values as floating times because they are not fixed to a specific incremental time value. The specific time when 11th June starts or ends in Britain may actually be on Friday or Sunday in other countries, because their clocks are set differently, but the date of the event is always referred to as Saturday 11 June. Instead, they represent a nominal time which is described in the same way in all time zones around the world.įor example, Saturday 11 June 2016 happens to be the date of the British Queen's official 90th birthday. Some time values are not mapped to a specific moment in incremental time. The actual moment in time of noon indicated in this way will vary for people only a few miles apart. Observed time of noon is when the sun is at its highest point in the sky. The term observed time refers to a different method of ascertaining the time that relies on physical observation of things such as the sun, moon or stars, and is not really used these days. It's also possible to convert that to a wall time in another location, such as San Francisco, where someone looking at their computer's time display at the same time would have seen Fri 10 Jun 22:10. If I apply knowledge about how that time relates to UTC (in my case, adjusting by one hour to account for British Summer Time) I can convert that to the incremental time 1465621816590. So, for example, while writing the previous section I may have glanced at the time displayed by my computer and seen Sat 11 Jun 06:10. However, incremental time is not usually seen directly by users, but is typically mapped to a field-based time format for interchange or for human consumption.

Most programming languages and operating environments provide or use incremental time for working with time values. Other systems use different units and/or epochs.įor example, at one point while writing this paragraph the incremental time for JavaScript in my browser was 1465621816590 (ie. Java (and many other systems) count time as the number of milliseconds since midnight (00:00 a.m.) on Januin UTC (less all of the intervening leap seconds). Incremental time is a way of representing time in computers that is based on a progression of fixed integer units that increase monotonically from a specific point in time (called the 'epoch'). The formats described by the ISO 8601 standard are field-based. Field-based times are also typically tied to a specific calendar (such as the Gregorian calendar). Contrast this with an alternative way to express the same time, 1465621816590, which is not field-based and is rather hard to read.įield-based times may or may not be tied to either UTC or the local time zone – or they may be indeterminate. When you write a time using a field-based format you divide the date and/or time into separate field values such as year, month, day, hour, minute, second, etc. UTC is often indicated in field-based formats using Z.

Among other things, it provides a common baseline for converting between incremental and local time. Universal Coordinated Time (UTC) is the basis for modern timekeeping. There are some subtle differences between the two, but none that the average person would notice. UTC is also known as GMT (Greenwich Mean Time). The document Working with Time Zones provides greater detail on each of these topics, as well as guidelines on handling of times and dates on the Web.

This article gently introduces the concepts behind some basic terminology you will encounter when reading other articles that deal with time zones and handling of dates and times on the Web.

Gmt time zone definition