JavaScript: Find whether or not the date is in daylights savings time

JavaScript Datetime: Exercise-38 with Solution

Daylight Savings Check

Write a JavaScript function to find out whether or not the date is daylight savings time.

Test Data :
dt = new Date();
console.log(daylights_savings(dt));
1

Sample Solution:

JavaScript Code:

// Define a function daylights_savings that takes a Date object dt as input
function daylights_savings(dt) 
{ 
  // Initialize a variable dst to null to store the daylight savings time offset
  var dst = null;
  // Iterate over the months of the year
  for (var i = 0; i < 12; ++i) 
    {
      // Create a new Date object for the current year and month
      var d = new Date(dt.getFullYear(), i, 1);
      // Retrieve the timezone offset for the current date
      var offset = dt.getTimezoneOffset();

      // Check if the daylight savings time offset has been set
      if (dst === null) 
        dst = offset;
      // If the current offset is less than the previously stored offset, update the dst variable
      else if (offset < dst) 
         {
           dst = offset; 
           // Exit the loop if a lower offset is found
           break;
         } 
      // If the current offset is greater than the previously stored offset, exit the loop
      else if (offset > dst) 
        break;
         }
// Return 1 if the timezone offset of the provided date matches the daylight savings offset, else return 0
return (dt.getTimezoneOffset() == dst) | 0;
}

// Create a new Date object representing the current date and time
dt = new Date(); 
// Output whether the current date and time is during daylight savings time (1 for yes, 0 for no)
console.log(daylights_savings(dt)); 

// Create a new Date object representing November 1, 1989
dt = new Date(1989, 10, 1); 
// Output whether November 1, 1989, is during daylight savings time (1 for yes, 0 for no)
console.log(daylights_savings(dt));

Output:

1
1

Explanation:

In the exercise above,

• The function takes a Date object "dt" as input.
• It initializes a variable 'dst' to null to store the daylight savings time offset.
• It iterates over the months of the year using a loop.
• Inside the loop:
• It creates a new Date object "d" for the current year and month.
• It retrieves the timezone offset for the current date using dt.getTimezoneOffset().
• It checks if the daylight savings time offset 'dst' has been set:
• If not, it sets 'dst' to the current offset.
• If the current offset is less than the previously stored offset, it updates 'dst' to the current offset and exits the loop.
• If the current offset is greater than the previously stored offset, it exits the loop.
• Finally, it compares the timezone offset of the provided date "dt" with the daylight savings offset 'dst'. If they match, it returns 1; otherwise, it returns 0.
• The code then creates a new Date object representing the current date and time (dt = new Date()) and outputs whether the current date and time is during daylight savings time (1 for yes, 0 for no) using console.log(daylights_savings(dt)).
• Similarly, it creates a new Date object representing November 1, 1989, and outputs whether November 1, 1989, is during daylight savings time.

Flowchart:

Live Demo:

See the Pen JavaScript - Find whether or not the date is in daylights savings time-date-ex-38 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript function that compares the standard timezone offset with the current offset to determine if DST is in effect.
• Write a JavaScript function that calculates the difference between winter and summer offsets to confirm daylight saving time.
• Write a JavaScript function that returns true if a given Date object is during daylight savings time and false otherwise.
• Write a JavaScript function that handles regions without DST and returns a custom message indicating no DST.

Improve this sample solution and post your code through Disqus.

Previous: Write a JavaScript function to get Timezone.
Next: Write a JavaScript function to get difference to Greenwich time (GMT) in hours.