21.3 The Class java.util.Date

The class Date provides a system-independent abstraction of dates and times, to a millisecond precision. Dates may be constructed from a year, month, date (day of month), hour, minute, and second; those six components and the day of the week, may be extracted; and dates may be compared and converted to a readable string.

public class Date {
	public Date();
	public Date(long time);
	public Date(int year, int month, int date);
	public Date(int year, int month, int date,
			int hours, int minutes);
	public Date(int year, int month, int date,
			int hours, int minutes, int seconds);
	public Date(String s) throws IllegalArgumentException;
	public String toString();
	public boolean equals(Object obj);
	public int hashCode();
	public int getYear();
	public void setYear(int year);
	public int getMonth();
	public void setMonth(int month);
	public int getDate();
	public void setDate(int date);
	public int getDay();
	public int getHours();
	public void setHours(int hours);
	public int getMinutes();
	public void setMinutes(int minutes);
	public int getSeconds();
	public void setSeconds(int seconds);
	public long getTime();
	public void setTime(long time);
	public boolean before(Date when);
	public boolean after(Date when);
	public String toLocaleString();
	public String toGMTString();
	public int getTimezoneOffset();
	public static long UTC(int year, int month, int date,
			int hours, int minutes, int seconds);
	public static long parse(String s)
		throws IllegalArgumentException;
}

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 = = 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 example, 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 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 GPS (the satellite-based Global Positioning System) 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, day of month, hours, minutes, and seconds values, the following representations are used:

• A year y is represented by the integer .
• A month is represented by an integer form 0 to 11; 0 is January, 1 is February, and so on; 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 AM is hour 0, and the hour from noon to 1 PM 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 will 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 will be interpreted as meaning February 1.

21.3.1 public Date()

This constructor initializes a newly created Date object so that it represents the instant of time that it was created, measured to the nearest millisecond.

21.3.2 public Date(long time)

This constructor initializes a newly created Date object so that it represents the instant of time that is time milliseconds after the standard base time known as "the epoch," namely 00:00:00 GMT on January 1, 1970. See also the method currentTimeMillis (§20.18.6) of class System.

21.3.3 public Date(int year, int month, int date)

This constructor initializes a newly created Date object so that it represents midnight at the beginning of the day specified by the year, month, and date arguments, in the local time zone. Thus, it has the same effect as the constructor call (§21.3.5):

Date(year, month, date, 0, 0, 0) 

21.3.4 public Date(int year, int month, int date,
int hours, int minutes)

This constructor initializes a newly created Date object so that it represents the instant at the start of the minute specified by the year, month, date, hours, and minutes arguments, in the local time zone. Thus, it has the same effect as the constructor call (§21.3.5):

Date(year, month, date, hours, minutes, 0)

21.3.5 public Date(int year, int month, int date,
int hours, int minutes, int seconds)

This constructor initializes a newly created Date object so that it represents the instant at the start of the second specified by the year, month, date, hours, minutes, and seconds arguments, in the local time zone.

21.3.6 public Date(String s)
throws IllegalArgumentException

This constructor initializes a newly created Date object so that it represents the date and time indicated by the string s, which is interpreted as if by the parse method (§21.3.31).

21.3.7 public String toString()

This Date object is converted to a String of the form:

"dow mon dd hh:mm:ss zzz yyyy"

where:

• dow is the day of the week (Sun, Mon, Tue, Wed, Thu, Fri, Sat).
• mon is the month (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec).
• dd is the day of the month (01 through 31), as two decimal digits.
• hh is the hour of the day (00 through 23), as two decimal digits.
• mm is the minute within the hour (00 through 59), as two decimal digits.
• ss is the second within the minute (00 through 61), as two decimal digits.
• zzz is the time zone (and may reflect daylight saving time). Standard time zone abbreviations include those recognized by the method parse (§21.3.31). If time zone information is not available, then zzz is empty-that is, it consists of no characters at all.
• yyyy is the year, as four decimal digits.

See also methods toLocaleString (§21.3.27) and toGMTString (§21.3.28).

Overrides the toString method of Object (§20.1.2).

21.3.8 public boolean equals(Object obj)

The result is true if and only if the argument is not null and is a Date object that represents the same point in time, to the millisecond, as this Date object. Thus two Date objects are equal if and only if the getTime method (§21.3.23) returns the same long value from both.

Overrides the equals method of Object (§20.1.3).

21.3.9 public int hashCode()

The result is the exclusive OR of the two halves of the primitive long value returned by the getTime method (§21.3.23). That is, the hash code is the value of the expression:

(int)(this.getTime()^(this.getTime()>>>32))

Overrides the hashCode method of Object (§20.1.4).

21.3.10 public int getYear()

The returned value is the result of subtracting 1900 from the year that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.

21.3.11 public void setYear(int year)

This Date object is modified so that it represents a point in time within the specified year, with the month, date, hour, minute, and second the same as before, as interpreted in the local time zone. (Of course, if the date was February 29, for example, and the year is set to a non-leap year, then the new date will be treated as if it were on March 1.)

21.3.12 public int getMonth()

The returned value is a number (0 through 11) representing the month that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.

21.3.13 public void setMonth(int month)

This Date object is modified so that it represents a point in time within the specified month, with the year, date, hour, minute, and second the same as before, as interpreted in the local time zone. If the date was October 31, for example, and the month is set to June, then the new date will be treated as if it were on July 1, because June has only 30 days.

21.3.14 public int getDate()

The returned value is a number (1 through 31) representing day of the month that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.

21.3.15 public void setDate(int date)

This Date object is modified so that it represents a point in time within the specified day of the month, with the year, month, hour, minute, and second the same as before, as interpreted in the local time zone.If the date was April 30, for example, and the date is set to 31, then it will be treated as if it were on May 1, because April has only 30 days.

21.3.16 public int getDay()

The returned value (0 = Sunday, 1 = Monday, 2 = Tuesday, 3 = Wednesday, 4 = Thursday, 5 = Friday, 6 = Saturday) represents the day of the week that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.

21.3.17 public int getHours()

The returned value is a number (0 through 23) representing the hour within the day that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.

21.3.18 public void setHours(int hours)

This Date object is modified so that it represents a point in time within the specified hour of the day, with the year, month, date, minute, and second the same as before, as interpreted in the local time zone.

21.3.19 public int getMinutes()

The returned value is a number (0 through 59) representing the minute within the hour that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.

21.3.20 public void setMinutes(int minutes)

This Date object is modified so that it represents a point in time within the specified minute of the hour, with the year, month, date, hour, and second the same as before, as interpreted in the local time zone.

21.3.21 public int getSeconds()

The returned value is a number (0 through 61) representing the second within the minute that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.

21.3.22 public void setSeconds(int seconds)

This Date object is modified so that it represents a point in time within the specified second of the minute, with the year, month, date, hour, and minute the same as before, as interpreted in the local time zone.

21.3.23 public long getTime()

This method returns the time represented by this Date object, represented as the distance, measured in milliseconds, of that time from the epoch (00:00:00 GMT on January 1, 1970).

21.3.24 public void setTime(long time)

This Date object is modified so that it represents a point in time that is time milliseconds after the epoch (00:00:00 GMT on January 1, 1970).

21.3.25 public boolean before(Date when)

The result is true if and only if the instant represented by this Date object is strictly earlier than the instant represented by when.

21.3.26 public boolean after(Date when)

The result is true if and only if the instant represented by this Date object is strictly later than the instant represented by when.

21.3.27 public String toLocaleString()

This Date object is converted to a String of an implementation-dependent form. The general intent is that the form should be familiar to the user of the Java application, wherever it may happen to be running. The intent is comparable to that of the %c format supported by the strftime function of ISO C.

See also methods toString (§21.3.7) and toGMTString (§21.3.28).

21.3.28 public String toGMTString()

This Date object is converted to a String of length 23 or 24 of the form:

"d mon yyyy hh:mm:ss GMT"

where:

• d is the day of the month (1 through 31), as one or two decimal digits.
• mon is the month (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec).
• yyyy is the year, as four decimal digits.
• hh is the hour of the day (00 through 23), as two decimal digits.
• mm is the minute within the hour (00 through 59), as two decimal digits.
• ss is the second within the minute (00 through 61), as two decimal digits.
• GMT is exactly the ASCII letters "GMT" to indicate Greenwich Mean Time.

The result does not depend on the local time zone.

See also methods toString (§21.3.7) and toLocaleString (§21.3.27).

21.3.29 public int getTimezoneOffset()

This method returns the offset, measured in minutes, for the local time zone relative to UTC that is appropriate for the time represented by this Date object.

For example, in Massachusetts, five time zones west of Greenwich:

new Date(96, 1, 14).getTimezoneOffset() returns 300

because on February 14, 1996, standard time (Eastern Standard Time) is in use, which is offset five hours from UTC; but:

new Date(96, 5, 1).getTimezoneOffset() returns 240

because on May 1, 1996, daylight saving time (Eastern Daylight Time) is in use, which is offset only four hours from UTC.

This method produces the same result as if it computed:

(this.getTime() - UTC(this.getYear(),
							this.getMonth(),
							this.getDate(),
							this.getHours(),
							this.getMinutes(),
							this.getSeconds())) / (60 * 1000)

21.3.30 public static long UTC(int year, int month, int date,
int hours, int minutes, int seconds)

The arguments are interpreted as a year, month, day of the month, hour of the day, minute within the hour, and second within the minute, exactly as for the Date constructor of six arguments (§21.3.5), except that the arguments are interpreted relative to UTC rather than to the local time zone. The time indicated is returned represented as the distance, measured in milliseconds, of that time from the epoch (00:00:00 GMT on January 1, 1970).

21.3.31 public static long parse(String s)
throws IllegalArgumentException

An attempt is made to interpret the string s as a representation of a date and time. If the attempt is successful, the time indicated is returned represented as the distance, measured in milliseconds, of that time from the epoch (00:00:00 GMT on January 1, 1970). If the attempt fails, an IllegalArgumentException is thrown.

The string s is processed from left to right, looking for data of interest.

Any material in s that is within the ASCII parenthesis characters ( and ) is ignored. Parentheses may be nested. Otherwise, the only characters permitted within s are these ASCII characters:

abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789,+-:/

and whitespace characters (§20.5.19).

A consecutive sequence of decimal digits is treated as a decimal number:

• If a number is preceded by + or - and a year has already been recognized, then the number is a time-zone offset. If the number is less than 24, it is an offset measured in hours. Otherwise, it is regarded as an offset in minutes, expressed in 24-hour time format without punctuation. A preceding + means an eastward offset and a preceding - means a westward offset. Time zone offsets are always relative to UTC (Greenwich). Thus, for example, -5 occurring in the string would mean "five hours west of Greenwich" and +0430 would mean "four hours and thirty minutes east of Greenwich." It is permitted for the string to specify GMT, UT, or UTC redundantly-for example, GMT-5 or utc+0430.
• If a number is greater than 70, it is regarded as a year number. It must be followed by a space, comma, slash, or end of string. If it is greater than 1900, then 1900 is subtracted from it.
• If the number is followed by a colon, it is regarded as an hour, unless an hour has already been recognized, in which case it is regarded as a minute.
• If the number is followed by a slash, it is regarded as a month (it is decreased by 1 to produce a number in the range 0 to 11), unless a month has already been recognized, in which case it is regarded as a day of the month.
• If the number is followed by whitespace, a comma, a hyphen, or end of string, then if an hour has been recognized but not a minute, it is regarded as a minute; otherwise, if a minute has been recognized but not a second, it is regarded as a second; otherwise, it is regarded as a day of the month.

A consecutive sequence of letters is regarded as a word and treated as follows:

• A word that matches AM, ignoring case, is ignored (but the parse fails if an hour has not been recognized or is less than 1 or greater than 12).
• A word that matches PM, ignoring case, adds 12 to the hour (but the parse fails if an hour has not been recognized or is less than 1 or greater than 12).
• Any word that matches any prefix of SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, or SATURDAY, ignoring case, is ignored. For example, sat, Friday, TUE, and Thurs are ignored.
• Otherwise, any word that matches any prefix of JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, or DECEMBER, ignoring case, and considering them in the order given here, is recognized as specifying a month and is converted to a number (0 to 11). For example, aug, Sept, april, and NOV are recognized as months. So is Ma, which is recognized as MARCH, not MAY.
• Any word that matches GMT, UT, or UTC, ignoring case, is treated as referring to UTC.
• Any word that matches EST, CST, MST, or PST, ignoring case, is recognized as referring to the time zone in North America that is five, six, seven, or eight hours west of Greenwich, respectively. Any word that matches EDT, CDT, MDT, or PDT, ignoring case, is recognized as referring to the same time zone, respectively, during daylight saving time. (In the future, this method may be upgraded to recognize other time zone designations.)

Once the entire string s has been scanned, it is converted to a time result in one of two ways. If a time zone or time-zone offset has been recognized, then the year, month, day of month, hour, minute, and second are interpreted in UTC (§21.3.30) and then the time-zone offset is applied. Otherwise, the year, month, day of month, hour, minute, and second are interpreted in the local time zone.