ICU TimeZone Classes


A time zone is a system that is used for relating local times in different geographical areas to one another. For example, in the United States, Pacific Time is three hours earlier than Eastern Time; when it's 6 P.M. in San Francisco, it's 9 P.M. in Brooklyn. To make things simple, instead of relating time zones to one another, all time zones are related to a common reference point.

For historical reasons, the reference point is Greenwich, England. Local time in Greenwich is referred to as Greenwich Mean Time, or GMT. (This is similar, but not precisely identical, to Universal Coordinated Time, or UTC. We use the two terms interchangeably in ICU since ICU does not concern itself with either leap seconds or historical behavior.) Using this system, Pacific Time is expressed as GMT-8:00, or GMT-7:00 in the summer. The offset -8:00 indicates that Pacific Time is obtained from GMT by adding -8:00, that is, by subtracting 8 hours.

The offset differs in the summer because of daylight savings time, or DST. At this point it is useful to define three different flavors of local time:

  • Standard Time
    Standard Time is local time without a daylight savings time offset. For example, in California, standard time is GMT-8:00; that is, 8 hours before GMT.

  • Daylight Savings Time
    Daylight savings time is local time with a daylight savings time offset. This offset is typically one hour, but is sometimes less. In California, daylight savings time is GMT-7:00. Daylight savings time is observed in most non-equatorial areas.

  • Wall Time
    Wall time is what a local clock on the wall reads. In areas that observe daylight savings time for part of the year, wall time is either standard time or daylight savings time, depending on the date. In areas that do not observe daylight savings time, wall time is equivalent to standard time.

Time Zones in ICU

ICU supports time zones through two classes:

  • TimeZone
    TimeZone is an abstract base class that defines the time zone API. This API supports conversion between GMT and local time.

  • SimpleTimeZone
    SimpleTimeZone is a concrete subclass of TimeZone that implements the standard time zones used today internationally.

Timezone classes are related to UDate, the Calendar classes, and the DateFormat classes.

Timezone Class in ICU

TimeZone is an abstract base class. It defines common protocol for a hierarchy of classes. This protocol includes:

  • A programmatic ID, for example, "America/Los_Angeles". This ID is used to call up a specific real-world time zone. It corresponds to the IDs defined in the standard Olson data used by UNIX systems, and has the format continent/city or ocean/city.

  • A raw offset. This is the difference, in milliseconds, between a time zone's standard time and GMT. Positive raw offsets are east of Greenwich.

  • Factory methods and methods for handling the default time zone.

  • Display name methods.

  • An API to compute the difference between local wall time and GMT.

Factory Methods and the Default Timezone

The TimeZone factory method createTimeZone() creates and returns a TimeZone object given a programmatic ID. The user does not know what the class of the returned object is, other than that it is a subclass of TimeZone.

The createAvailableIDs() methods return lists of the programmatic IDs of all zones known to the system. These IDs may then be passed to createTimeZone() to create the actual time zone objects. ICU maintains a comprehensive list of current international time zones, as derived from the Olson data.

TimeZone maintains a static time zone object known as the default time zone. This is the time zone that is used implicitly when the user does not specify one. ICU attempts to match this to the host OS time zone. The user may obtain a clone of the default time zone by calling createDefault() and may change the default time zone by calling setDefault() or adoptDefault().

Display Name

When displaying the name of a time zone to the user, use the display name, not the programmatic ID. The display name is returned by the getDisplayName() method. A time zone may have three display names:

  • Generic name, such as "Pacific Time". Currently, this is not supported by ICU.

  • Standard name, such as "Pacific Standard Time".

  • Daylight savings name, such as "Pacific Daylight Time".

Furthermore, each of these names may be LONG or SHORT. The SHORT form is typically an abbreviation, e.g., "PST", "PDT".

In addition to being available directly from the TimeZone API, the display name is used by the date format classes to format and parse time zones.

getOffset() API

TimeZone defines the API getOffset() by which the caller can determine the difference between local time and GMT. This is a pure virtual API, so it is implemented in the concrete subclasses of TimeZone.

Updating the Time Zone Data

Time zone data changes often in response to governments around the world changing their local rules and the areas where they apply. ICU derives its tz data from the IANA Time Zone Database.

The ICU project publishes updated timezone resource data in response to IANA updates, and these can be used to patch existing ICU installations. Several update strategies are possible, depending on the ICU version and configuration.

  • ICU4J: Use the time zone update utility.

  • ICU4C 54 and newer: Drop in the binary update files.

  • ICU4C 36 and newer: the best update strategy will depend on how ICU data loading is configured for the specific ICU installation.

    • Data is loaded from a .dat package file: replace the time zone resources in the .dat file using the icupkg tool.
    • Data is loaded from a .dll or .so shared library: obtain the updated sources for the tz resources and rebuild the data library.
    • Data is loaded from individual files: drop in the updated binary .res files. 

The ICU Data section of this user guide gives more information on how ICU loads resources.

The  ICU resource files required for time zone data updates are posted at . The required resource files for ICU version 44 and newer are

  • zoneinfo64.res
  • windowsZones.res
  • timezoneTypes.res
  • metaZones.res

ICU4C TZ update of a .dat Package File

For ICU configurations that load data from a .dat package file, replace the time zone resources in that file.
  1. Download the new .res files from<iana tz version number>/44/<platform directory>/.
    • "44" is the directory for updates to ICU version 4.4 and newer.
    • <platform directory> is "le" for little endian processors, including all Intel processors.
    • <platform directory> is "be" for big endian processors, including IBM Power and Sparc.
    • <platform directory> is "ee" for IBM mainframes using EBCDIC character sets.

  2. Check that the tool "icupkg" is available. If not already on your system, you can get it by downloading and building ICU, following the instructions in the ReadMe file included in the download. Alternatively, on many Linux systems, "apt-get install icu-devtools" will install the tool.

  3. Locate the .dat file to be updated, and do the update. The commands below are for a .dat file named icudt55l.dat.
      icupkg -a zoneinfo64.res  icudt55l.dat
      icupkg -a windowsZones.res  icudt55l.dat
      icupkg -a timezoneTypes.res  icudt55l.dat
      icupkg -a metaZones.res  icudt55l.dat

In ICU versions older than 4.4 some of the time zone resources have slightly different names. The update procedure is the same, but substitute the names found in the desired download directory - 42, 40, 38 or 36. 

ICU4C TZ Update with Drop-in .res files (ICU 54 and newer)

With this approach, the four individual .res files are dropped in any convenient location in the file system, and ICU is given an absolute path to the directory containing them. For the time zone resources only, ICU will check this directory first when loading data. This approach will work even when all other ICU data loading is from a shared library or .dat file.

There are two ways to specify the directory:
  • At ICU build time, by defining the C pre-processor variable U_TIMEZONE_FILES_DIR to the run time path to the directory containing the .res files.
  • At run time, by setting the environment variable ICU_TIMEZONE_FILES_DIR to the absolute path of the directory containing the .res files.
If both are defined, the environment variable ICU_TIMEZONE_FILES_DIR take precedence. If either is defined, the time zone directory will be checked first, meaning that time zone resource files placed there will override time zone resources that may exist in other ICU data locations.

To do the update, download the .res files appropriate for the platform, as described for the .dat file update above, and copy them into the time zone res file directory.

ICU4C TZ update when ICU is configured for individual files

If the ICU-using application sets an ICU data path (or can be changed to set one), then the time zone .res file can be placed there. Download the files as described above and copy them to the specified directory. See the ICU Data page of the user guide for more information about the ICU data path.

ICU4C TZ update when ICU data is built into a shared library

  1. Set up the environment necessary to rebuild your specific configuration of ICU.

  2. Download the .txt file sources for the updated resources from<iana tz version number>/44

  3. Copy the downloaded .txt files into the ICU sources for your installation, in the subdirectory  source/data/misc/

  4. Rebuid ICU.

  5. Copy the freshly built ICU data shared library to the desired destination.

There are too many possible platform variations to be more specific about how to rebuild ICU4C in these instructions. See the ReadMe file included with the ICU sources for general information on building ICU.

Update the time zone data for ICU4J

The  ICU4J Time Zone Update Update Utility automates the process of updating ICU4J jar files with the latest time zone data. Instructions for use are here.

The updater will work with ICU version 3.4.2 and newer.

Sample Code

See the Date and Time Zone Examples subpage.