Lære Daylight Saving Time and Edge Cases | Advanced Time Handling and Modern Tools

Daylight saving time (DST) is a system in which clocks are set forward by one hour during warmer months to extend evening daylight. In regions that observe DST, clocks typically "spring forward" in the spring and "fall back" in the autumn. This practice can significantly impact date and time calculations in JavaScript. When performing arithmetic with dates—such as adding hours or days—crossing a DST boundary can result in unexpected results, because the local time may suddenly jump forward or backward by an hour. This can cause issues in scheduling, logging, and any application logic that relies on precise time intervals.


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// Suppose DST starts on March 14, 2021, at 2:00 AM in New York (America/New_York)
// Clocks move forward from 2:00 AM to 3:00 AM

// Create a date just before the DST transition
const beforeDST = new Date('2021-03-14T01:30:00-05:00'); // 1:30 AM EST (UTC-5)
console.log('Before DST:', beforeDST.toString());

// Add 1 hour
const afterDST = new Date(beforeDST.getTime() + 60 * 60 * 1000);
console.log('After adding 1 hour:', afterDST.toString());

To avoid DST-related bugs in your applications, consider several strategies:

Perform all critical date arithmetic in UTC rather than local time, which eliminates ambiguity caused by DST transitions;
Store dates and times in UTC in your databases and only convert to local time for display purposes;
When scheduling events or calculating intervals that must be consistent regardless of time zone, always use UTC methods such as Date.UTC() and the getUTC* and setUTC* methods;
For applications that must operate across multiple time zones, use clear documentation and consistent conventions for handling time zones and DST, and thoroughly test any logic that operates near DST boundaries.

Using these strategies helps ensure that your application's time calculations remain accurate and predictable, even when crossing daylight saving boundaries.

Further Reading: Daylight Saving Time Resources

For more guidance on handling daylight saving time in global applications, consult the following resources:

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              12345678910
            
// Suppose DST starts on March 14, 2021, at 2:00 AM in New York (America/New_York)
// Clocks move forward from 2:00 AM to 3:00 AM

// Create a date just before the DST transition
const beforeDST = new Date('2021-03-14T01:30:00-05:00'); // 1:30 AM EST (UTC-5)
console.log('Before DST:', beforeDST.toString());

// Add 1 hour
const afterDST = new Date(beforeDST.getTime() + 60 * 60 * 1000);
console.log('After adding 1 hour:', afterDST.toString());