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Java.util.Date Class

Date Class

public class Date
extends Object
implements Serializable, Cloneable, Comparable<Date>

The class Date represents a specific instant in time, with millisecond precision.

Prior to JDK 1.1, the class Date had two additional functions. It allowed the interpretation of dates as year, month, day, hour, minute, and second values. It also allowed the formatting and parsing of date strings. Unfortunately, the API for these functions was not amenable to internationalization. As of JDK 1.1, the Calendar class should be used to convert between dates and time fields and the DateFormat class should be used to format and parse date strings. The corresponding methods in Date are deprecated.

Although 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.

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.

Constructor Summary:

Name Description
Date() Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.
Date(long date) Allocates a Date object and initializes it to represent the specified number of milliseconds since the standard base time known as "the epoch", namely January 1, 1970, 00:00:00 GMT.

Method Summary:

Name Type Description
after(Date when) boolean Tests if this date is after the specified date.
before(Date when) void Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
clone() Object Return a copy of this object.
compareTo(Date anotherDate) int Compares two Dates for ordering.
equals(Object obj) boolean Compares two dates for equality.
from(Instant instant) static Date Obtains an instance of Date from an Instant object.
getTime() long Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.
hashCode() int Returns a hash code value for this object.
setTime(long time) void Sets this Date object to represent a point in time that is time milliseconds after January 1, 1970 00:00:00 GMT.
toInstant() Instant Converts this Date object to an Instant.
toString() String Converts this Date object to a String of the form:

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