net.time4j.scale

Interface UniversalTime

All Superinterfaces:

UnixTime

All Known Implementing Classes:

Moment, ZonalDateTime

public interface UniversalTime
extends UnixTime

Defines a time point respective coordinate on the universal timeline as the count of seconds relative to UTC epoch [1972-01-01T00:00:00Z] in the UTC-timezone (Greenwich meridian) and a nanosecond as part of last second.

The abbreviation "UTC" stands for the term "Corrected Universal Time". This term relates to the timezone UTC+00:00 as reference for all timestamps. This API uses as measurement system a combination of seconds (hold in a long-primitive) and nanoseconds as fraction of last second (hold in an int-primitive). In such defined system all time points of type TimePoint can be directly compared with each other which implement this interface. The time of the technical introduction of UTC in radio signals on the date 1972-01-01 is determined by Time4J as UTC-epoch. Before this epoch the second is just the 86400th part of a mean solar day. After the UTC-epoch the atomic time is valid with the new definition of the second as SI-second based on the count of periods in a caesium atom at a given frequency transition.

Generally the rules of univeral time are those of the gregorian calendar - with the exception of leapseconds. UTC and ISO-8601 (for the representation) take into account leapseconds which were introduced in order to always keep the difference between an atomic clock and the mean solar time UT1 within the range of 0.9 seconds. The solar time UT1 is a mean average time due to the anomalies associated with the irregular rotation of earth around the own axis where every day consists of 86400 seconds having a variable length. Therefore these seconds are no SI-seconds. This API reads UTC-leapseconds from a table and defines the universal time in SI-seconds since the begin of UTC-epoch [1972-01-01T00:00:00Z] including a nanosecond part (mostly only precise to milliseconds). Taking out the leapseconds and by transforming to the POSIX-epoch 1970-01-01 yields the POSIX-time where every minute always have 60 seconds (on the base of one calendar day equal to 86400 seconds).

The UTC-standard defined so far was officially introduced on the date 1972-01-01. Befor this UTC-epoch there are no leapseconds. The consequence is that Time4J interpretes all historical timestamps before 1972 as UT1-time - with reference to mean solar day on Greenwich meridian. Correspondingly the definiton of the second changes with the begin of UTC-epoch to atomic based time. The definiton of the calendar day changes, too. For all UT1-timestamps the calendar day is identical with the mean solar day which can yield the true solar day using the equation of time (astronomical formula). But with start of UTC a calendar day consists of 86400 SI-seconds, in rare cases of 86401 or (until now only theoretical) 86399 SI-seconds. This definition of the calendar day is already now no longer exact equal to the mean solar day. The leapseconds only guarantee an approximation of the calendar day to the mean solar day such that the difference will never go beyond 0.9 seconds.

There is a debate initiated by USA to abolish leapseconds. If this might happen then Time4J would start a new section on the universal timeline after UT1 and UTC - with a probably new name. The new section will surely be based on SI-seconds but indeed stop to count any leapseconds. This interface would then mutate to a pure atomic scale without any reference to the mean solar day. The deviation of the atomic day from the mean solar day is in year 2013 already about 25 SI-seconds. The ITU-conference in January 2012 just decided to postpone the decision about the proposal to abolish leapseconds first in year 2015. ITU is an international organization for controlling the radio signals and telecommunication standards. Currently ITU manages the UTC standard, too. Time4J stores timestamps using the concept of this interface as followed:

Legend

Section	Name	Second definition	Details
before 1972-01-01T00:00:00Z	UT1	mean solar second	A mean solar day corresponds to always 86400 seconds with variable length.
1972-01-01T00:00:00Z until day X	UTC (with leapseconds)	SI-second based on atomic clocks	A calendar day consists in most cases of 86400 SI-seconds sometimes also of 86401 or 86399 SI-seconds. The mechanism of leapseconds is responsible for keeping the difference between the calendar day and the mean solar day within the range of 0.9 SI-seconds. Due to the impossibility to make long-term-predictions about the earth rotation the day X is about 6 months after current date or last leapsecond in the future. The institute IERS is responsible for announcing leapseconds around 6 months in advance. A second precision is not given for any kind of time calculations if the calculation includes the future further than 6 months.
after day X	IT ??? (International Time ??? - without leapseconds)	SI-second based on atomic clocks	Future leapseconds are no longer defined here. Consequently the calendar day based on 86400 SI-seconds will increasingly deviate from the mean solar day.

If leapseconds in Time4J are switched off per configuration then UTC will practically mutate to POSIX with the exception of the different epoch date (two years between epochs). Calculations of time deltas in SI-seconds and fractional seconds will get a small error after UTC-epoch which can normally be ignored in standard business use-cases (for example banks do not care about seconds when calculating day-based interest rates).

Arguments for keeping leapseconds alive:

• ASTRONOMY: The difference between the atomic time and the mean solar time UT1 is a physical fact which cannot simply be denied. The UT1-time is more important in civil life because human beings are accustomed to expect the highest sun around noon. Without the UTC-mechanism of leapseconds the time would come where noon occurs in the night which is not acceptable for anybody.
• PRECISION: An API can only guarantee precision around leapsecond events if the take into account leapseconds. This is especially true for time differences in seconds or fractions of seconds. All clocks nowadays used in practise finally use atomic clocks as reference clocks and hence indirectly use SI-seconds. Even if they themselves don't use atomic clock mechanisms (quartz, spring works etc.) so they are adapted to atomic clocks in any way. This is not always neglectable because the used measurement system in Java goes down to nanoseconds. This is the Java-based reasoning why UniversalTime uses UTC as universal time scale. Simultaneously the limitation to leapseconds on the section from 1972 and later takes into account that before this epoch there is no precision in seconds due to technical and practical reasons. Even today nanosecond precision is not realistic with all kinds of computer clocks or the NTP-protocol and only exists here for database support (SQL-TIMESTAMP-support, databases often generate nano-timestamps as primary keys by help of random-based numbers or a simple numerical counter). But after 1972 it is also valid to say that for example relative inaccurate computer clocks can adapted manually to UTC-time and are sufficiently sensible for leapseconds near leapsecond events if it is just about second precision.
• STANDARDS: Another important reason is that UTC is currently the actual world standard in civil life and important technical standards like ISO-8601 or internet protocols like RFC 3339. This API has the goal to follow these standards as much as possible. Leapseconds are rare but nevertheless present in civil life if they happen. They are always published in the broad mass media.
• DEFINITION OF SECOND: UTC based on atomic clocks ensures that every second has exactly the same length what is not true for the usual mean solar time (UT1) which is more present in civil life. Variations like the expired proposal UTC-SLS have never played any role and try to hide leapseconds by introducing leap milliseconds. Latter case does not really favor any practical use case. Such proposals counteract the basic idea of UTC to define seconds with constant and same length.

Further links for definition of UTC:

• Wikipedia
• History of leapseconds
• Actual announcements of leapseconds

Author:

Meno Hochschild

See Also:

LeapSeconds

Specification:

Implementations are required to be immutable.

Method Summary

Modifier and Type	Method and Description
long	getElapsedTime(TimeScale scale) Represents this timestamp as elpased seconds on given time scale.
int	getNanosecond(TimeScale scale) Represents the nanosecond part on the given time scale.
boolean	isLeapSecond() Queries if this time point is within a positive leapsecond.

Methods inherited from interface net.time4j.base.UnixTime

getNanosecond, getPosixTime

Method Detail

getElapsedTime

long getElapsedTime(TimeScale scale)

Represents this timestamp as elpased seconds on given time scale.

The method getPosixTime() inherited from UnixTime is equivalent to getElapsedTime(TimeScale.POSIX) and relates to the UNIX-epoch 1970-01-01. The time scale UTC starts two years later however and also counts leapseconds.

Parameters:

scale - time scale reference

Returns:

elapsed seconds in given time scale

Throws:

IllegalArgumentException - if this instance is out of range for given time scale

getNanosecond

int getNanosecond(TimeScale scale)

Represents the nanosecond part on the given time scale.

The method with the same name and without argument inherited from super interface UnixTime is identical to this method if the time scale is either POSIX or UTC.

Parameters:

scale - time scale reference

Returns:

nanosecond fraction in given time scale

Throws:

IllegalArgumentException - if this instance is out of range for given time scale

isLeapSecond

boolean isLeapSecond()

Queries if this time point is within a positive leapsecond.

If the support for UTC-leapseconds is switched off per configuration then this method will always yield false.

Returns:

true if this instance represents a positive leap second else false

See Also:

LeapSeconds.isEnabled()