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Design Concepts

The purpose of this library is to offer an efficient support for time zone handling in the smallest package.

  • Lightweight - nothing else is included. Thus serving well for other date & time libraries, but also for applications, which do not manipulate dates.
  • Tiny - use packed time zone data, unpacked on demand. Compromise between loading time and being ready to use immediately.
  • Reliable - generated from the fresh official time zone database version 2019a. Canonical time zone names, aliases, UTC offsets, and daylight-saving time changes.
  • Customizable - named exports and functions divided to separate modules allow tree-shaking. Alternative time zone data can be supplied to reduce the library size.

Table of Contents

Basics

The interface is built around two value types:

  1. UNIX timestamp for a date in UTC
  2. Time object for a date in an arbitrary time zone

UNIX Timestamp

The UNIX timestamp can be used to create a native Date object instance, or it can be obtained from it by calling date.prototype.getTime. It serves well as a common way how to represent a date in UTC. (The UNIX timestamp used in this library is the epoch time in milliseconds, not in seconds, to be compatible with the interface of the native Date object.)

The UNIX timestamp is returned by the function getUnixTime converting from an arbitrary time zone to UTC.

Time Object

The time object is used to carry a date and information about an arbitrary time zone. The native Date object cannot include the time zone information with the date.

{
  year: 2018,
  month: 9,
  day: 25,
  hours: 9,
  minutes: 30,
  seconds: 45,
  milliseconds: 273,
  epoch: 1537860645273,
  dayOfWeek: 2,
  zone: {
    abbreviation: 'CEST',
    offset: -120
  }
}

The following properties are expected in an incomplete time object, usually set from the user input or by the function parseZonedTime. The properties seconds and milliseconds are optional and will be considered as set to zero, if they are missing.

  • year (Number) - full year number, as returned by Date.prototype.getFullYear
  • month (Number) - month number starting from 1 (January); not as returned by Date.prototype.getMonth, which returns a zero-based month index
  • day (Number) - day of the month number starting from 1, as returned by Date.prototype.getDate
  • hours (Number) - elapsed hours within a day starting from 0, as returned by Date.prototype.getHours
  • minutes (Number) - elapsed minutes within n hour starting from 0, as returned by Date.prototype.getMinutes
  • seconds (Number) - elapsed seconds within a minute starting from 0, as returned by Date.prototype.getSeconds
  • milliseconds (Number) - elapsed milliseconds within a second starting from 0, as returned by Date.prototype.getMilliseconds

The following properties are added to a complete time object, usually set by conversion function getZonedTime and setZonedTime.

  • dayOfWeek (Number) - day of the week number starting from 0 (Sunday), as returned by Date.prototype.getDay
  • epoch (Number) - elapsed milliseconds from the UNIX epoch (UTC), as returned by Date.prototype.getTime
  • zone (Object) - information about the time zone for the day stored in the time object
  • zone.abbreviation (String) - time zone abbreviation for the day stored in the time object
  • zone.offset (Number) - difference between UTC and the time zone of the time object in minutes, as Date.prototype.getTimezoneOffset would return it, if it supported arbitrary time zones

The property zone.offset has the opposite sign, than the time zone offset formatted in displayable strings. For example, its value for "CEST+02:00" is -120. If this offset is subtracted from the UNIX timestamp in UTC, you will get the epoch time in the given time zone. Similarly, if you have an epoch time in a specific time zone, adding the zone.offset to it will convert it to UTC.