Daylight Saving Time Math: The 23-Hour Day, the 25-Hour Day, and Duration Errors

Elapsed Time Across a Transition

A night-shift worker clocks in at 10:00 p.m. on Saturday, March 7 and out at 6:00 a.m. on Sunday, March 8 in the United States. By the clock, that is 8 hours, but because 2:00 to 3:00 a.m. does not exist, only 7 hours actually elapsed. On Saturday, October 31, the same shift from 10:00 p.m. to 6:00 a.m. lasts 9 hours, since the 1:00 to 2:00 a.m. hour occurs twice. Payroll rules typically require paying for the hours actually worked, so the pay differs by an hour on those nights.

To compute durations correctly, convert both times to UTC (which has no daylight saving), subtract, and convert back.

Adding a Day vs. Adding 24 Hours

In calendar arithmetic, 'add one day' means the same clock time on the next date, while 'add 24 hours' means exactly 86,400 seconds later. Across a DST change, these differ by an hour. A meeting at 9:00 a.m. on Saturday, March 7, plus one day, is 9:00 a.m. on Sunday, March 8, which is only 23 hours later in elapsed time. Software libraries handle this differently: some treat days as 24-hour blocks and others as calendar days. Test your tools with a date near a transition.

Recurring events, such as a daily 9:00 a.m. call, stay at the same local time across DST, but their UTC time shifts by an hour, which matters for people in regions that do not change clocks.

Scheduling Across Regions

The United States and Europe change on different dates. For about two to three weeks in spring (March 8 to March 29, 2026) and one week in autumn (October 25 to November 1, 2026), the time difference between New York and London is 4 hours instead of the usual 5. Meetings that were comfortable can suddenly clash with a school run or the end of the working day. Regions near the equator generally do not observe DST, so their offset from DST regions changes twice a year.

When scheduling across zones, state the time in UTC or include the city name and date, and check a world clock tool for the specific date.

Alarms, Ages, and Other Edge Cases

Times that do not exist, like 2:30 a.m. on the spring transition date, cannot be scheduled in local time, and software may shift them or throw errors. Times that occur twice, like 1:30 a.m. on the autumn date, are ambiguous without an offset. For age and anniversary calculations, DST does not matter because dates are counted in days, not hours. For medical dosing schedules or machine monitoring, use elapsed time rather than clock time.

Practical Tips

Store timestamps in UTC and convert to local time only for display. Use IANA time zone names, like America/New_York, not fixed offsets. In spreadsheets, avoid mixing local times that cross a DST boundary in duration formulas without adjusting. Verify results manually around the change dates, and remind teams about meeting times in the week before the clock change.