Java Reference
In-Depth Information
You might think, because we have been adding leap seconds to UTC, UTC should be ahead of TAI. That is not
true. Adding a leap second to UTC makes that hour on UTC scale 61 seconds long instead of 60 seconds. TAI is a
continuous time scale; it keeps ticking all the time. When UTC completes the 61st second of the hour, TAI has moved
to the first second in the next hour. Thus, UTC lags TAI when a leap second is added to it. Similar logic, but in reverse
order, applies when a leap second is subtracted from UTC. If, at any time in future, the leap seconds added to and
subtracted from UTC become equal, UTC and TAI will read the same time.
UTC represents the time of day at the prime meridian (zero degree longitude) on Earth, which passes through
Greenwich, England. UTC is based on a 24-hour clock with the day starting at 00 hours midnight. UTC is also
known as Zulu time. The ISO-8601 standard uses the letter Z as the UTC of day designator; for example, the UTC of
day 19 minutes and 23 seconds past 15 hours is written as 15:19:23Z.
You're not done with UTC yet! I will discuss two more versions of UTC: Simplified UTC and UTC with Smoothed
Leap Seconds (UTC-SLS).
Humans are used to understanding a solar day in terms of a 24-hour period: each hour consisting of 60 minutes
and each minute consisting of 60 seconds. A solar day consists of 86400 seconds. On the UTC scale, a solar day may
also consist of 86399 or 86401 seconds because of a leap second. For easier understanding for the common user, most
computer systems ignore the leap seconds on the UTC scale. The UTC scale that ignores the leap seconds is called a
simplified-UTC scale.
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to meet expectations of most users, the new Java Date-time api uses simplified UtC, where leap seconds are
ignored, making all days having the same number of 86400 seconds.
Tip
When the leap second is added or subtracted in UTC, it creates a gap or overlap of one second in the time scale
at the end of the day. UTC-SLS is a proposed standard for handling a UTC leap second. Instead of introducing a leap
second at the end of a day, UTC-SLS proposes to perform a smooth adjustment of 1 second over last 1000 seconds of
the day by changing the rate of the clock by 0.1%. On a day when a leap second is added to UTC, UTC-SLS will make
the last 1000 seconds of that day 1001 milliseconds long; thus, reducing the rate of the UTC-SLS clock from 23:43:21 to
24:00:00 by 0.1%. On a day when a leap second is added to UTC, UTC-SLS will make the last 1000 seconds of that day
as 999 milliseconds long; thus, increasing the rate of the UTC-SLS clock from 23:43:19 to 24:00:00 by 0.1%.
Finally, there are proposals to have universal and monotonous civil time by getting rid of the leap seconds from
UTC. Some have also proposed to replace the UTC leap seconds by leap hours!
Time Zones and Daylight Saving Time
When it was midnight of April 20, 2012 UTC, what local time was it in New Delhi, India and Chicago, USA? It was 5:30
in the morning of April 20, 2012 in New Delhi, India, and 7:00 in the evening of April 19, 2012 in Chicago, USA. How
do we determine local time at a place? Wouldn't it be nice to have only one time for the entire world? If it is midnight
UTC, it is midnight everywhere in the world. Maybe this would have been a good idea in the past as human minds are
capable of getting used to new ideas over time through practice. Local time in a region is set such that a day starts at
00 hours, which is midnight. Therefore, 00 hours is midnight in New Delhi as well as Chicago.
Geographically, the world may be divided into 24 longitudinal bands, each covering a 15-degree range of
longitude starting at the prime meridian; each band represents a one-hour time zone. The area covered by a time
zone will observe the same time.
Humans are divided more politically than geographically. In this world, our political differences always override
the geographical similarity! Sometimes an imaginary border separating two countries or states makes people observe
different times on each side of the border.
