Date coding in java

Java Date and Time

Java does not have a built-in Date class, but we can import the java.time package to work with the date and time API. The package includes many date and time classes. For example:

Class	Description
LocalDate	Represents a date (year, month, day (yyyy-MM-dd))
LocalTime	Represents a time (hour, minute, second and nanoseconds (HH-mm-ss-ns))
LocalDateTime	Represents both a date and a time (yyyy-MM-dd-HH-mm-ss-ns)
DateTimeFormatter	Formatter for displaying and parsing date-time objects

If you don’t know what a package is, read our Java Packages Tutorial.

Display Current Date

To display the current date, import the java.time.LocalDate class, and use its now() method:

Example

import java.time.LocalDate; // import the LocalDate class public class Main < public static void main(String[] args) < LocalDate myObj = LocalDate.now(); // Create a date object System.out.println(myObj); // Display the current date >> 

Display Current Time

To display the current time (hour, minute, second, and nanoseconds), import the java.time.LocalTime class, and use its now() method:

Example

import java.time.LocalTime; // import the LocalTime class public class Main < public static void main(String[] args) < LocalTime myObj = LocalTime.now(); System.out.println(myObj); >> 

Display Current Date and Time

To display the current date and time, import the java.time.LocalDateTime class, and use its now() method:

Example

import java.time.LocalDateTime; // import the LocalDateTime class public class Main < public static void main(String[] args) < LocalDateTime myObj = LocalDateTime.now(); System.out.println(myObj); >> 

Formatting Date and Time

The «T» in the example above is used to separate the date from the time. You can use the DateTimeFormatter class with the ofPattern() method in the same package to format or parse date-time objects. The following example will remove both the «T» and nanoseconds from the date-time:

Example

import java.time.LocalDateTime; // Import the LocalDateTime class import java.time.format.DateTimeFormatter; // Import the DateTimeFormatter class public class Main < public static void main(String[] args) < LocalDateTime myDateObj = LocalDateTime.now(); System.out.println("Before formatting: " + myDateObj); DateTimeFormatter myFormatObj = DateTimeFormatter.ofPattern("dd-MM-yyyy HH:mm:ss"); String formattedDate = myDateObj.format(myFormatObj); System.out.println("After formatting: " + formattedDate); >> 

The ofPattern() method accepts all sorts of values, if you want to display the date and time in a different format. For example:

Value	Example	Tryit
yyyy-MM-dd	«1988-09-29»	Try it »
dd/MM/yyyy	«29/09/1988»	Try it »
dd-MMM-yyyy	«29-Sep-1988»	Try it »
E, MMM dd yyyy	«Thu, Sep 29 1988»	Try it »

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Class Date

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. An interesting source of further information is the United States Naval Observatory (USNO):

and the material regarding «Systems of Time» at:

which has descriptions of various different time systems including UT, UT1, and UTC.

• 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

Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.

As of JDK version 1.1, replaced by Calendar.set(year + 1900, month, date) or GregorianCalendar(year + 1900, month, date) .

As of JDK version 1.1, replaced by Calendar.set(year + 1900, month, date, hrs, min) or GregorianCalendar(year + 1900, month, date, hrs, min) .

As of JDK version 1.1, replaced by Calendar.set(year + 1900, month, date, hrs, min, sec) or GregorianCalendar(year + 1900, month, date, hrs, min, sec) .

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.

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