Daylight Saving Time creates unique challenges for developers. Times can be ambiguous, invalid, or skip entirely. Understanding DST transitions helps you avoid bugs that only appear twice a year.
The Spring Forward Problem
When clocks spring forward, an hour is skipped. 2:00 AM becomes 3:00 AM instantly. Times like 2:30 AM simply do not exist on that day. If a user schedules something for 2:30 AM on DST transition day, your app must handle this gracefully.
The Fall Back Problem
When clocks fall back, an hour is repeated. 1:30 AM occurs twice—once in daylight time, once in standard time. If you only store local time without offset, you cannot distinguish between these two moments. Always store UTC or include offset information.
Scheduling Across DST
A daily meeting at 9:00 AM should stay at 9:00 AM local time regardless of DST—this is wall clock time scheduling. A reminder 24 hours before an event should use absolute time (UTC). Know which type of scheduling your feature needs.
Testing DST Logic
DST bugs are hard to catch because transitions happen only twice yearly. Test with dates specifically around DST transitions. Use past transition dates for reproducible tests. Test in multiple timezones—US, EU, and Australia transition on different dates.