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The Date Class

• The Date class represents a date and time, accurate to the nearest millisecond.
• Print the current date and time:
  

  var d = new Date( );
  document.writeln(d);
  // Output:
  Sun Feb 21 2021 10:11:22 GMT-0600 (Central Standard Time)
  

• Print an arbitrary date. If not specified the time is midnight (00:00:00). Notice that for the month, 2 represents March, not February, because JavaScript month indices are zero-based.
  

  var d = new Date(1978, 2, 18);
  document.writeln(d);
  // Output:
  Sat Mar 18 1978 00:00:00 GMT-0600 (Central Standard Time)
  

• Print an arbitrary date and time. The time is 24-hour time.
  

  var d = new Date(1978, 2, 18, 23, 59, 58);
  document.writeln(d);
  // Output: 
  Sat Mar 18 1978 23:59:58 GMT-0600 (Central Standard Time) 
  

• The Date class has methods getYear, getMonth, getDay, getHours, getMinutes, getSeconds, and getTime.
• The first six of these methods are what you expect:
  

  var d = new Date( );
  var yr = d.getFullYear( );
  var mon = d.getMonth( );
  var day = d.getDate( );
  var hr = d.getHours( );
  var min = d.getMinutes( );
  var sec = d.getSeconds( );
  document.writeln(`${yr} ${mon} ${day} ${hr} ${min} ${sec}`);
  // Output: 2021 1 21 10 31 56
  

• The getTime method is surprising. It is the number of milliseconds since midnight, Jan 1, 1970, which is the start of "Unix Time." Try it out:
  

  var d = new Date( );
  document.writeln(d.getTime( ));
  

• Try this script. Can you figure out why you don't get 0 for output, since this is the Unix start date?
  

  var d = new Date(1970,0,1);
  document.writeln(d.getTime( ));
  // Output: 21600000
  

  Answer: If the timezone were GMT (Greenwich Mean Time), the output would be 0. However, the time is for Central Standard Time, which is 6 hours before GMT. The date and time 6 hours before midnight on Jan 1, 1970 is Dec 31, 1969:
  

  var d = new Date(1969, 11, 31, 18, 0, 0);
  document.writeln(d.getTime( ));
  // Output: 0
  

• Modify the Clock Example so that it obtains the current time every second from a JavaScript Date object. This is more accurate than implementing your own Clock class.