CODEWITHSUNDEEP
java
Peter
Peter

Reputation: 41

Where does Java get the time?

I'm wondering where Java get's it's time. Does it update based on system or it takes the system time and goes from there at run time.

For example, I will use the java.util.Date to take the time. When I call a method to get the time from this class, where does the time come from and based on my calls to get the time does it check it from the system or does it call an internal clock in the vm.

Upvotes: 2

Views: 953

Answers (3)

Juned Ahsan
Juned Ahsan

Reputation: 68715

Right from the javadoc:

the Date class is intended to reflect coordinated universal time (UTC), it may not do so exactly, depending on the host environment of the Java Virtual Machine. 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 always on December 31 or June 30. For example, the last minute of the year 1995 was 61 seconds long, thanks to an added leap second. Most computer clocks are not accurate enough to be able to reflect the leap-second distinction.

Some computer standards are defined in terms of Greenwich mean time (GMT), which is equivalent to universal time (UT). 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 purposes is an invisibly fine hair to split. Because the earth's rotation is not uniform (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 the satellite-based global positioning system (GPS) is synchronized to UTC but is not adjusted for leap seconds. An interesting source of further information 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, date, 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 from 0 to 11; 0 is January, 1 is February, and so forth; thus 11 is December. A date (day of month) is represented by an integer from 1 to 31 in the usual manner. An hour is represented by an integer from 0 to 23. Thus, the hour from midnight to 1 a.m. is hour 0, and the hour from noon to 1 p.m. is hour 12. A minute is represented by an integer from 0 to 59 in the usual manner. A second is represented by an integer from 0 to 61; the values 60 and 61 occur only for leap seconds and even then only in Java implementations that actually track leap seconds correctly. Because of the manner in which leap seconds are currently introduced, it is extremely unlikely that two leap seconds will occur in the same minute, but this specification follows the date and time conventions for ISO C. In all cases, arguments given to methods for these purposes need not fall within the indicated ranges; for example, a date may be specified as January 32 and is interpreted as meaning February 1.

Upvotes: 3

nos
nos

Reputation: 229118

It comes from the underlying OS, the conventional JVMs does not keep an internal running clock themselves

Upvotes: 2

stacker
stacker

Reputation: 68962

See the constructor of java.util.Date

public Date() {
    this(System.currentTimeMillis());
}

Upvotes: 8

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