TimeZone

public abstract class TimeZone extends Object
implements Serializable Cloneable Freezable<TimeZone>

[icu enhancement] ICU's replacement for TimeZone. Methods, fields, and other functionality specific to ICU are labeled '[icu]'.

TimeZone represents a time zone offset, and also computes daylight savings.

Typically, you get a TimeZone using getDefault() which creates a TimeZone based on the time zone where the program is running. For example, for a program running in Japan, getDefault creates a TimeZone object based on Japanese Standard Time.

You can also get a TimeZone using getTimeZone(String) along with a time zone ID. For instance, the time zone ID for the U.S. Pacific Time zone is "America/Los_Angeles". So, you can get a U.S. Pacific Time TimeZone object with:

 TimeZone tz = TimeZone.getTimeZone("America/Los_Angeles");
 
You can use the getAvailableIDs() method to iterate through all the supported time zone IDs, or getCanonicalID method to check if a time zone ID is supported or not. You can then choose a supported ID to get a TimeZone. If the time zone you want is not represented by one of the supported IDs, then you can create a custom time zone ID with the following syntax:
 GMT[+|-]hh[[:]mm]
 
For example, you might specify GMT+14:00 as a custom time zone ID. The TimeZone that is returned when you specify a custom time zone ID uses the specified offset from GMT(=UTC) and does not observe daylight saving time. For example, you might specify GMT+14:00 as a custom time zone ID to create a TimeZone representing 14 hours ahead of GMT (with no daylight saving time). In addition, getCanonicalID can also be used to normalize a custom time zone ID.

For compatibility with JDK 1.1.x, some other three-letter time zone IDs (such as "PST", "CTT", "AST") are also supported. However, their use is deprecated because the same abbreviation is often used for multiple time zones (for example, "CST" could be U.S. "Central Standard Time" and "China Standard Time"), and the Java platform can then only recognize one of them.

See Also

Nested Class Summary

enum TimeZone.SystemTimeZoneType [icu] System time zone type constants used by filtering zones in getAvailableIDs(SystemTimeZoneType, String, Integer)  

Constant Summary

int GENERIC_LOCATION [icu] A style specifier for getDisplayName() indicating a long name derived from the timezone's fallback name, such as "United States (Los Angeles)."
int LONG A style specifier for getDisplayName() indicating a long name, such as "Pacific Standard Time."
int LONG_GENERIC [icu] A style specifier for getDisplayName() indicating a long generic name, such as "Pacific Time."
int LONG_GMT [icu] A style specifier for getDisplayName() indicating a long name derived from the timezone's offset, such as "GMT-08:00."
int SHORT A style specifier for getDisplayName() indicating a short name, such as "PST."
int SHORT_COMMONLY_USED [icu] A style specifier for getDisplayName() indicating a short name derived from the timezone's short standard or daylight timezone name ignoring commonlyUsed, such as "PDT."
int SHORT_GENERIC [icu] A style specifier for getDisplayName() indicating a short generic name, such as "PT."
int SHORT_GMT [icu] A style specifier for getDisplayName() indicating a short name derived from the timezone's offset, such as "-0800."
int TIMEZONE_ICU [icu] A time zone implementation type indicating ICU's own TimeZone used by getTimeZone.
int TIMEZONE_JDK [icu] A time zone implementation type indicating the TimeZone used by getTimeZone.
String UNKNOWN_ZONE_ID [icu] The time zone ID reserved for unknown time zone.

Field Summary

public static final TimeZone GMT_ZONE [icu] The immutable GMT (=UTC) time zone.
public static final TimeZone UNKNOWN_ZONE [icu] The immutable (frozen) "unknown" time zone.

Public Constructor Summary

TimeZone()
Default constructor.

Public Method Summary

Object
clone()
Overrides clone.
TimeZone
cloneAsThawed()
Provides for the clone operation.
static int
countEquivalentIDs(String id)
[icu] Returns the number of IDs in the equivalency group that includes the given ID.
boolean
equals(Object obj)
Overrides equals.
TimeZone
freeze()
Freezes the object.
static String[]
getAvailableIDs()
Return a new String array containing all system TimeZone IDs.
static String[]
getAvailableIDs(int rawOffset)
Return a new String array containing all system TimeZone IDs with the given raw offset from GMT.
static Set<String>
getAvailableIDs(TimeZone.SystemTimeZoneType zoneType, String region, Integer rawOffset)
[icu] Returns a set of time zone ID strings with the given filter conditions.
static String[]
getAvailableIDs(String country)
Return a new String array containing all system TimeZone IDs associated with the given country.
static String
getCanonicalID(String id)
[icu] Returns the canonical system time zone ID or the normalized custom time zone ID for the given time zone ID.
static String
getCanonicalID(String id, boolean[] isSystemID)
[icu] Returns the canonical system time zone ID or the normalized custom time zone ID for the given time zone ID.
int
getDSTSavings()
Returns the amount of time to be added to local standard time to get local wall clock time.
static TimeZone
getDefault()
Gets the default TimeZone for this host.
String
getDisplayName(boolean daylight, int style, Locale locale)
Returns a name of this time zone suitable for presentation to the user in the specified locale.
final String
getDisplayName()
Returns a name of this time zone suitable for presentation to the user in the default DISPLAY locale.
final String
getDisplayName(ULocale locale)
Returns a name of this time zone suitable for presentation to the user in the specified locale.
String
getDisplayName(boolean daylight, int style, ULocale locale)
Returns a name of this time zone suitable for presentation to the user in the specified locale.
final String
getDisplayName(Locale locale)
Returns a name of this time zone suitable for presentation to the user in the specified locale.
final String
getDisplayName(boolean daylight, int style)
Returns a name of this time zone suitable for presentation to the user in the default DISPLAY locale.
static String
getEquivalentID(String id, int index)
Returns an ID in the equivalency group that includes the given ID.
static TimeZone
getFrozenTimeZone(String ID)
Gets the TimeZone for the given ID.
String
getID()
Gets the ID of this time zone.
static String
getIDForWindowsID(String winid, String region)
[icu] Converts a Windows time zone ID to an equivalent system time zone ID for a region.
void
getOffset(long date, boolean local, int[] offsets)
Returns the time zone raw and GMT offset for the given moment in time.
int
getOffset(long date)
Returns the offset of this time zone from UTC at the specified date.
abstract int
getOffset(int era, int year, int month, int day, int dayOfWeek, int milliseconds)
Gets the time zone offset, for current date, modified in case of daylight savings.
abstract int
getRawOffset()
Gets unmodified offset, NOT modified in case of daylight savings.
static String
getRegion(String id)
[icu] Returns the region code associated with the given system time zone ID.
static String
getTZDataVersion()
[icu] Returns the time zone data version currently used by ICU.
static TimeZone
getTimeZone(String ID, int type)
Gets the TimeZone for the given ID and the timezone type.
static TimeZone
getTimeZone(String ID)
Gets the TimeZone for the given ID.
static String
getWindowsID(String id)
[icu] Converts a system time zone ID to an equivalent Windows time zone ID.
boolean
hasSameRules(TimeZone other)
Returns true if this zone has the same rule and offset as another zone.
int
hashCode()
Overrides hashCode.
abstract boolean
inDaylightTime(Date date)
Queries if the given date is in daylight savings time in this time zone.
boolean
isFrozen()
Determines whether the object has been frozen or not.
boolean
observesDaylightTime()
Queries if this time zone is in daylight saving time or will observe daylight saving time at any future time.
void
setID(String ID)
Sets the time zone ID.
abstract void
setRawOffset(int offsetMillis)
Sets the base time zone offset to GMT.
abstract boolean
useDaylightTime()
Queries if this time zone uses daylight savings time.

Inherited Method Summary

Constants

public static final int GENERIC_LOCATION

[icu] A style specifier for getDisplayName() indicating a long name derived from the timezone's fallback name, such as "United States (Los Angeles)."

Constant Value: 7

public static final int LONG

A style specifier for getDisplayName() indicating a long name, such as "Pacific Standard Time."

See Also
Constant Value: 1

public static final int LONG_GENERIC

[icu] A style specifier for getDisplayName() indicating a long generic name, such as "Pacific Time."

See Also
Constant Value: 3

public static final int LONG_GMT

[icu] A style specifier for getDisplayName() indicating a long name derived from the timezone's offset, such as "GMT-08:00."

See Also
Constant Value: 5

public static final int SHORT

A style specifier for getDisplayName() indicating a short name, such as "PST."

See Also
Constant Value: 0

public static final int SHORT_COMMONLY_USED

[icu] A style specifier for getDisplayName() indicating a short name derived from the timezone's short standard or daylight timezone name ignoring commonlyUsed, such as "PDT."

Constant Value: 6

public static final int SHORT_GENERIC

[icu] A style specifier for getDisplayName() indicating a short generic name, such as "PT."

See Also
Constant Value: 2

public static final int SHORT_GMT

[icu] A style specifier for getDisplayName() indicating a short name derived from the timezone's offset, such as "-0800."

See Also
Constant Value: 4

public static final int TIMEZONE_ICU

[icu] A time zone implementation type indicating ICU's own TimeZone used by getTimeZone.

Constant Value: 0

public static final int TIMEZONE_JDK

[icu] A time zone implementation type indicating the TimeZone used by getTimeZone.

Constant Value: 1

public static final String UNKNOWN_ZONE_ID

[icu] The time zone ID reserved for unknown time zone.

Constant Value: "Etc/Unknown"

Fields

public static final TimeZone GMT_ZONE

[icu] The immutable GMT (=UTC) time zone. Its ID is "Etc/GMT".

public static final TimeZone UNKNOWN_ZONE

[icu] The immutable (frozen) "unknown" time zone. It behaves like the GMT/UTC time zone but has the UNKNOWN_ZONE_ID = "Etc/Unknown". getTimeZone(String) returns a mutable clone of this time zone if the input ID is not recognized.

Public Constructors

public TimeZone ()

Default constructor. (For invocation by subclass constructors, typically implicit.)

Public Methods

public Object clone ()

Overrides clone.

Returns
  • a copy of this object.

public TimeZone cloneAsThawed ()

Provides for the clone operation. Any clone is initially unfrozen.

public static int countEquivalentIDs (String id)

[icu] Returns the number of IDs in the equivalency group that includes the given ID. An equivalency group contains zones that have the same GMT offset and rules.

The returned count includes the given ID; it is always >= 1 for valid IDs. The given ID must be a system time zone. If it is not, returns zero.

Parameters
id a system time zone ID
Returns
  • the number of zones in the equivalency group containing 'id', or zero if 'id' is not a valid system ID

public boolean equals (Object obj)

Overrides equals.

Parameters
obj the object to compare this instance with.
Returns
  • true if the specified object is equal to this Object; false otherwise.

public TimeZone freeze ()

Freezes the object.

Returns
  • the object itself.

public static String[] getAvailableIDs ()

Return a new String array containing all system TimeZone IDs. These IDs (and only these IDs) may be passed to get() to construct the corresponding TimeZone object.

Returns
  • an array of all system TimeZone IDs

public static String[] getAvailableIDs (int rawOffset)

Return a new String array containing all system TimeZone IDs with the given raw offset from GMT. These IDs may be passed to get() to construct the corresponding TimeZone object.

Parameters
rawOffset the offset in milliseconds from GMT
Returns
  • an array of IDs for system TimeZones with the given raw offset. If there are none, return a zero-length array.

public static Set<String> getAvailableIDs (TimeZone.SystemTimeZoneType zoneType, String region, Integer rawOffset)

[icu] Returns a set of time zone ID strings with the given filter conditions.

Note:A Set returned by this method is immutable.

Parameters
zoneType The system time zone type.
region The ISO 3166 two-letter country code or UN M.49 three-digit area code. When null, no filtering done by region.
rawOffset An offset from GMT in milliseconds, ignoring the effect of daylight savings time, if any. When null, no filtering done by zone offset.
Returns
  • an immutable set of system time zone IDs.

public static String[] getAvailableIDs (String country)

Return a new String array containing all system TimeZone IDs associated with the given country. These IDs may be passed to get() to construct the corresponding TimeZone object.

Parameters
country a two-letter ISO 3166 country code, or null to return zones not associated with any country
Returns
  • an array of IDs for system TimeZones in the given country. If there are none, return a zero-length array.

public static String getCanonicalID (String id)

[icu] Returns the canonical system time zone ID or the normalized custom time zone ID for the given time zone ID.

Parameters
id The input time zone ID to be canonicalized.
Returns
  • The canonical system time zone ID or the custom time zone ID in normalized format for the given time zone ID. When the given time zone ID is neither a known system time zone ID nor a valid custom time zone ID, null is returned.

public static String getCanonicalID (String id, boolean[] isSystemID)

[icu] Returns the canonical system time zone ID or the normalized custom time zone ID for the given time zone ID.

Parameters
id The input time zone ID to be canonicalized.
isSystemID When non-null boolean array is specified and the given ID is a known system time zone ID, true is set to isSystemID[0]
Returns
  • The canonical system time zone ID or the custom time zone ID in normalized format for the given time zone ID. When the given time zone ID is neither a known system time zone ID nor a valid custom time zone ID, null is returned.

public int getDSTSavings ()

Returns the amount of time to be added to local standard time to get local wall clock time.

The default implementation always returns 3600000 milliseconds (i.e., one hour) if this time zone observes Daylight Saving Time. Otherwise, 0 (zero) is returned.

If an underlying TimeZone implementation subclass supports historical Daylight Saving Time changes, this method returns the known latest daylight saving value.

Returns
  • the amount of saving time in milliseconds

public static TimeZone getDefault ()

Gets the default TimeZone for this host. The source of the default TimeZone may vary with implementation.

Returns
  • a default TimeZone.

public String getDisplayName (boolean daylight, int style, Locale locale)

Returns a name of this time zone suitable for presentation to the user in the specified locale. If the display name is not available for the locale, then this method returns a string in the localized GMT offset format such as GMT[+-]HH:mm.

Parameters
daylight if true, return the daylight savings name.
style the output style of the display name. Valid styles are SHORT, LONG, SHORT_GENERIC, LONG_GENERIC, SHORT_GMT, LONG_GMT, SHORT_COMMONLY_USED or GENERIC_LOCATION.
locale the locale in which to supply the display name.
Returns
  • the human-readable name of this time zone in the given locale or in the default locale if the given locale is not recognized.
Throws
IllegalArgumentException style is invalid.

public final String getDisplayName ()

Returns a name of this time zone suitable for presentation to the user in the default DISPLAY locale. This method returns the long generic name. If the display name is not available for the locale, a fallback based on the country, city, or time zone id will be used.

Returns
  • the human-readable name of this time zone in the default locale.
See Also

public final String getDisplayName (ULocale locale)

Returns a name of this time zone suitable for presentation to the user in the specified locale. This method returns the long name, not including daylight savings. If the display name is not available for the locale, a fallback based on the country, city, or time zone id will be used.

Parameters
locale the ulocale in which to supply the display name.
Returns
  • the human-readable name of this time zone in the given locale or in the default ulocale if the given ulocale is not recognized.

public String getDisplayName (boolean daylight, int style, ULocale locale)

Returns a name of this time zone suitable for presentation to the user in the specified locale. If the display name is not available for the locale, then this method returns a string in the localized GMT offset format such as GMT[+-]HH:mm.

Parameters
daylight if true, return the daylight savings name.
style the output style of the display name. Valid styles are SHORT, LONG, SHORT_GENERIC, LONG_GENERIC, SHORT_GMT, LONG_GMT, SHORT_COMMONLY_USED or GENERIC_LOCATION.
locale the locale in which to supply the display name.
Returns
  • the human-readable name of this time zone in the given locale or in the default locale if the given locale is not recognized.
Throws
IllegalArgumentException style is invalid.

public final String getDisplayName (Locale locale)

Returns a name of this time zone suitable for presentation to the user in the specified locale. This method returns the long generic name. If the display name is not available for the locale, a fallback based on the country, city, or time zone id will be used.

Parameters
locale the locale in which to supply the display name.
Returns
  • the human-readable name of this time zone in the given locale or in the default locale if the given locale is not recognized.

public final String getDisplayName (boolean daylight, int style)

Returns a name of this time zone suitable for presentation to the user in the default DISPLAY locale. If the display name is not available for the locale, then this method returns a string in the localized GMT offset format such as GMT[+-]HH:mm.

Parameters
daylight if true, return the daylight savings name.
style the output style of the display name. Valid styles are SHORT, LONG, SHORT_GENERIC, LONG_GENERIC, SHORT_GMT, LONG_GMT, SHORT_COMMONLY_USED or GENERIC_LOCATION.
Returns
  • the human-readable name of this time zone in the default locale.
See Also

public static String getEquivalentID (String id, int index)

Returns an ID in the equivalency group that includes the given ID. An equivalency group contains zones that have the same GMT offset and rules.

The given index must be in the range 0..n-1, where n is the value returned by countEquivalentIDs(id). For some value of 'index', the returned value will be equal to the given id. If the given id is not a valid system time zone, or if 'index' is out of range, then returns an empty string.

Parameters
id a system time zone ID
index a value from 0 to n-1, where n is the value returned by countEquivalentIDs(id)
Returns
  • the ID of the index-th zone in the equivalency group containing 'id', or an empty string if 'id' is not a valid system ID or 'index' is out of range

public static TimeZone getFrozenTimeZone (String ID)

Gets the TimeZone for the given ID. The instance of TimeZone returned by this method is immutable. Any methods mutate the instance(setID(String), setRawOffset(int)) will throw UnsupportedOperationException upon its invocation.

Parameters
ID the ID for a TimeZone, such as "America/Los_Angeles", or a custom ID such as "GMT-8:00". Note that the support of abbreviations, such as "PST", is for JDK 1.1.x compatibility only and full names should be used.
Returns
  • the specified TimeZone, or the UNKNOWN_ZONE if the given ID cannot be understood.
See Also

public String getID ()

Gets the ID of this time zone.

Returns
  • the ID of this time zone.

public static String getIDForWindowsID (String winid, String region)

[icu] Converts a Windows time zone ID to an equivalent system time zone ID for a region. For example, system time zone ID "America/Los_Angeles" is returned for input Windows ID "Pacific Standard Time" and region "US" (or null), "America/Vancouver" is returned for the same Windows ID "Pacific Standard Time" and region "CA".

Not all Windows time zones can be mapped to system time zones. When the input Windows time zone ID is unknown or unmappable to a system time zone, then this method returns null.

This implementation utilizes Zone-Tzid mapping data. The mapping data is updated time to time. To get the latest changes, please read the ICU user guide section Updating the Time Zone Data.

Parameters
winid A Windows time zone ID
region A region code, or null if no regional preference.
Returns
  • A system time zone ID mapped from the input Windows time zone ID, or null when the input ID is unknown or unmappable.

public void getOffset (long date, boolean local, int[] offsets)

Returns the time zone raw and GMT offset for the given moment in time. Upon return, local-millis = GMT-millis + rawOffset + dstOffset. All computations are performed in the proleptic Gregorian calendar. The default implementation in the TimeZone class delegates to the 8-argument getOffset().

Parameters
date moment in time for which to return offsets, in units of milliseconds from January 1, 1970 0:00 GMT, either GMT time or local wall time, depending on `local'.
local if true, `date' is local wall time; otherwise it is in GMT time.
offsets output parameter to receive the raw offset, that is, the offset not including DST adjustments, in offsets[0], and the DST offset, that is, the offset to be added to `rawOffset' to obtain the total offset between local and GMT time, in offsets[1]. If DST is not in effect, the DST offset is zero; otherwise it is a positive value, typically one hour.

public int getOffset (long date)

Returns the offset of this time zone from UTC at the specified date. If Daylight Saving Time is in effect at the specified date, the offset value is adjusted with the amount of daylight saving.

Parameters
date the date represented in milliseconds since January 1, 1970 00:00:00 GMT
Returns
  • the amount of time in milliseconds to add to UTC to get local time.

public abstract int getOffset (int era, int year, int month, int day, int dayOfWeek, int milliseconds)

Gets the time zone offset, for current date, modified in case of daylight savings. This is the offset to add *to* UTC to get local time.

Parameters
era the era of the given date.
year the year in the given date.
month the month in the given date. Month is 0-based. e.g., 0 for January.
day the day-in-month of the given date.
dayOfWeek the day-of-week of the given date.
milliseconds the millis in day in standard local time.
Returns
  • the offset to add *to* GMT to get local time.

public abstract int getRawOffset ()

Gets unmodified offset, NOT modified in case of daylight savings. This is the offset to add *to* UTC to get local time.

Returns
  • the unmodified offset to add *to* UTC to get local time.

public static String getRegion (String id)

[icu] Returns the region code associated with the given system time zone ID. The region code is either ISO 3166 2-letter country code or UN M.49 3-digit area code. When the time zone is not associated with a specific location, for example - "Etc/UTC", "EST5EDT", then this method returns "001" (UN M.49 area code for World).

Parameters
id the system time zone ID.
Returns
  • the region code associated with the given system time zone ID.
Throws
IllegalArgumentException if id is not a known system ID.

public static String getTZDataVersion ()

[icu] Returns the time zone data version currently used by ICU.

Returns
  • the version string, such as "2007f"
Throws
MissingResourceException if ICU time zone resource bundle is missing or the version information is not available.

public static TimeZone getTimeZone (String ID, int type)

Gets the TimeZone for the given ID and the timezone type.

Parameters
ID the ID for a TimeZone, such as "America/Los_Angeles", or a custom ID such as "GMT-8:00". Note that the support of abbreviations, such as "PST", is for JDK 1.1.x compatibility only and full names should be used.
type Time zone type, either TIMEZONE_ICU or TIMEZONE_JDK.
Returns
  • the specified TimeZone, or a mutable clone of the UNKNOWN_ZONE if the given ID cannot be understood or if the given ID is "Etc/Unknown".
See Also

public static TimeZone getTimeZone (String ID)

Gets the TimeZone for the given ID.

Parameters
ID the ID for a TimeZone, such as "America/Los_Angeles", or a custom ID such as "GMT-8:00". Note that the support of abbreviations, such as "PST", is for JDK 1.1.x compatibility only and full names should be used.
Returns
  • the specified TimeZone, or a mutable clone of the UNKNOWN_ZONE if the given ID cannot be understood or if the given ID is "Etc/Unknown".
See Also

public static String getWindowsID (String id)

[icu] Converts a system time zone ID to an equivalent Windows time zone ID. For example, Windows time zone ID "Pacific Standard Time" is returned for input "America/Los_Angeles".

There are system time zones that cannot be mapped to Windows zones. When the input system time zone ID is unknown or unmappable to a Windows time zone, then this method returns null.

This implementation utilizes Zone-Tzid mapping data. The mapping data is updated time to time. To get the latest changes, please read the ICU user guide section Updating the Time Zone Data.

Parameters
id A system time zone ID
Returns
  • A Windows time zone ID mapped from the input system time zone ID, or null when the input ID is unknown or unmappable.

public boolean hasSameRules (TimeZone other)

Returns true if this zone has the same rule and offset as another zone. That is, if this zone differs only in ID, if at all. Returns false if the other zone is null.

Parameters
other the TimeZone object to be compared with
Returns
  • true if the other zone is not null and is the same as this one, with the possible exception of the ID

public int hashCode ()

Overrides hashCode.

Returns
  • this object's hash code.

public abstract boolean inDaylightTime (Date date)

Queries if the given date is in daylight savings time in this time zone.

Parameters
date the given Date.
Returns
  • true if the given date is in daylight savings time, false, otherwise.

public boolean isFrozen ()

Determines whether the object has been frozen or not.

public boolean observesDaylightTime ()

Queries if this time zone is in daylight saving time or will observe daylight saving time at any future time.

The default implementation in this class returns true if useDaylightTime() or inDaylightTime(new Date()) returns true.

Note: This method was added for TimeZone compatibility support. The useDaylightTime() method only checks the last known rule(s), therefore it may return false even the zone observes daylight saving time currently. TimeZone added observesDaylightTime() to resolve the issue. In ICU, useDaylightTime() works differently. The ICU implementation checks if the zone uses daylight saving time in the current calendar year. Therefore, it will never return false if daylight saving time is currently used.

ICU's TimeZone subclass implementations override this method to support the same behavior with observesDaylightTime(). Unlike useDaylightTime(), the implementation does not take past daylight saving time into account, so that this method may return false even when useDaylightTime() returns true.

Returns
  • true if this time zone is in daylight saving time or will observe daylight saving time at any future time.

public void setID (String ID)

Sets the time zone ID. This does not change any other data in the time zone object.

Parameters
ID the new time zone ID.

public abstract void setRawOffset (int offsetMillis)

Sets the base time zone offset to GMT. This is the offset to add *to* UTC to get local time.

Parameters
offsetMillis the given base time zone offset to GMT.

public abstract boolean useDaylightTime ()

Queries if this time zone uses daylight savings time.

Returns
  • true if this time zone uses daylight savings time, false, otherwise.

    Note:The default implementation of ICU TimeZone uses the tz database, which supports historic rule changes, for system time zones. With the implementation, there are time zones that used daylight savings time in the past, but no longer used currently. For example, Asia/Tokyo has never used daylight savings time since 1951. Most clients would expect that this method to return false for such case. The default implementation of this method returns true when the time zone uses daylight savings time in the current (Gregorian) calendar year.