THE PACKAGE JAVA.UTIL

java.util.Date

21.3

Examples:

To print today s date:

System.out.println("today = " + new Date());

To find out the day of the week for some particular date, for example, January

16, 1963:

new Date(63, 0, 16).getDay()

While the

Date

 class is intended to reflect UTC (Coordinated Universal

Time), it may not do so exactly, depending on the host environment of the Java

system. Nearly all modern operating systems assume that 1 day = 24

60

60  =

86400 seconds in all cases. In UTC, however, about once every year or two there

is an extra second, called a  leap second.  The leap second is always added as the

last second of the day, and nearly always on December 31 or June 30. For exam 

ple, the last minute of the year 1995 was 61 seconds long, thanks to an added leap

second.

Most computer clocks are currently not accurate enough to be able to reflect

the leap second distinction. Some computer standards are defined in terms of

GMT (Greenwich Mean Time), which is equivalent to UT (Universal Time).

GMT is the  civil  name for the standard; UT is the  scientific  name for the same

standard. The distinction between UTC and UT is that UTC is based on an atomic

clock and UT is based on astronomical observations, which for all practical pur 

poses is an invisibly fine hair to split. Because the earth's rotation is not uni 

form it slows down and speeds up in complicated ways UT does not always

flow uniformly. Leap seconds are introduced as needed into UTC so as to keep

UTC within 0.9 seconds of UT1, which is a version of UT with certain corrections

applied. There are other time and date systems as well; for example, the time scale

used by GPS (the satellite based Global Positioning System) is synchronized to

UTC but is

not

 adjusted for leap seconds. An interesting source of further infor 

mation is the U. S. Naval Observatory, particularly the Directorate of Time at:

http://tycho.usno.navy.mil

and their definitions of  Systems of Time  at:

http://tycho.usno.navy.mil/systime.html

In all methods of class

Date

 that accept or return year, month, day of month,

hours, minutes, and seconds values, the following representations are used:

A year

y

 is represented by the integer

y

  1900 .

A month is represented by an integer form 0 to 11; 0 is January, 1 is February,

and so on; thus 11 is December.

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