java.util.Date - JDK Class to Measure Date and Time

This section provides a tutorial example on how to use the java.util.Date class to measure date and time. The Date class does not carry any calendar information.

Time usually refers to the hour, the minute and may be the second of a specific moment within a day. For example, if someone asks you: "What time is it?", you would answer: "It's nine thirty.", meaning that 9 hours and 30 minutes passed away since the beginning of the day.

Date usually refers to the calendar date of a specific day within a year. For example, if someone ask you: "What date is it today?", you would answer: "Today is October 25.", meaning that today is 21st day of the 10th month of this year.

So both terms: "time" and "date" are measurements of a specific moment. "Time" measures a specific moment with a unit of minute or second, using the beginning of the day as a reference point. "Date" measures a specific moment with a unit of day, using the beginning of the year as a reference point. Notice that these types of measurements are calendar and time zone dependent. At exactly the same moment, the values of "time" and "date" are different from one country to another.

In JDK, there is only one class, the Date class (java.util.Date), that measures a specific moment with a unit of millisecond, using a fixed moment of January 1, 1970, 00:00:00 GMT as a reference point.

The Date class measurement is calendar and time zone independent. So if you run the following code at exactly the same moment anywhere in the world, you will get exactly the same value:

   Date now = new Date(); // what time is it?
   long t = now.getTime(); 
   System.out.println("Time since 01-Jan-1970 00:00:00 GMT: " + 
      t + " milliseconds.");


Time since 01-Jan-1970 00:00:00 GMT: 1035236526708 milliseconds.

The Date class is very convenient for measuring a period of time:

   long t1 = (new Date()).getTime();
   // performing task XYZ
   long t2 = (new Date()).getTime();
   System.out.println("It took about " + ((t2-t1)/1000) + " seconds to"
      + " finish XYZ.");

However, the Date class does not provide any calendar and time zone related information. It requires the help of the Calendar class, see the next section.

The following is what you should remember about date and time in JDK:

Last update: 2014.

Table of Contents

 About This JDK Tutorial Book

 JDK (Java Development Kit)

 Java Date-Time API

Date, Time and Calendar Classes

java.util.Date - JDK Class to Measure Date and Time

 java.util.Calendar - The Abstract Calendar Class

 java.util.Calendar.add() - Calendar Manipulation Method

 Date and Time Object and String Conversion

 Number Object and Numeric String Conversion

 Locales, Localization Methods and Resource Bundles

 Calling and Importing Classes Defined in Unnamed Packages

 HashSet, Vector, HashMap and Collection Classes

 Character Set Encoding Classes and Methods

 Character Set Encoding Maps

 Encoding Conversion Programs for Encoded Text Files

 Java Logging

 Socket Network Communication

 Datagram Network Communication

 DOM (Document Object Model) - API for XML Files

 SAX (Simple API for XML)

 DTD (Document Type Definition) - XML Validation

 XSD (XML Schema Definition) - XML Validation

 XSL (Extensible Stylesheet Language)

 Message Digest Algorithm Implementations in JDK

 Private key and Public Key Pair Generation

 PKCS#8/X.509 Private/Public Encoding Standards

 Digital Signature Algorithm and Sample Program

 "keytool" Commands and "keystore" Files

 KeyStore and Certificate Classes

 Secret Key Generation and Management

 Cipher - Encryption and Decryption

 The SSL (Secure Socket Layer) Protocol

 SSL Socket Communication Testing Programs

 SSL Client Authentication

 HTTPS (Hypertext Transfer Protocol Secure)

 Outdated Tutorials


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