Daylight Saving Time & Time Math
What actually changes when the clocks shift — and what doesn't.
Quick answer
Daylight Saving Time shifts the wall clock by one hour, not elapsed time. The spring-forward day has 23 hours (an hour is skipped) and the fall-back day has 25 hours (an hour repeats). The real time between two instants never changes — only the labels on the clock do.
The 23-hour and 25-hour days
Spring forward → 23 hours
Clocks jump from 2:00 AM straight to 3:00 AM. That hour never appears on the clock, so the calendar day has only 23 wall-clock hours. You lose an hour of sleep.
Fall back → 25 hours
Clocks go from 2:00 AM back to 1:00 AM, so the 1:00–2:00 AM hour happens twice. The calendar day has 25 wall-clock hours. You gain an hour.
When the clocks change
| Region | Spring forward | Fall back |
|---|---|---|
| United States & Canada | 2nd Sunday of March | 1st Sunday of November |
| European Union & UK | Last Sunday of March | Last Sunday of October |
| Much of the world | No DST (e.g. most of Asia and Africa, Hawaii, most of Arizona) | |
Exact dates and rules come from the IANA time zone database — see methodology.
How DST affects time calculations
Elapsed time is safe. "8 hours from now" or "12 hours ago" always add or subtract real time, so the answer is correct even across a DST change — the displayed clock time just accounts for the shift. Our time from now and time ago tools use your time zone's rules automatically.
Wall-clock spans can surprise you. The clock time from 1:00 AM to 4:00 AM on a spring-forward night is only 2 real hours, not 3. On a fall-back night it is 4. When counting hours between two clock times across a change, count elapsed time.
The calendar day still "feels" like 24 hours in everyday use — see hours in a day for why the civil day is defined as 24 hours regardless.
Common pitfalls
- Scheduling a meeting for the skipped hour (2:00–3:00 AM in spring) — it does not exist that day.
- Payroll on the fall-back night: overnight shifts work a real 25th hour and should be paid for it.
- Assuming two regions change on the same date — the U.S. and EU shift on different Sundays, so the time difference between them changes for a week or two.
- Counting wall-clock hours across a change instead of elapsed time.
Frequently asked questions
Does Daylight Saving Time change how many hours are in a day?
The wall-clock day changes, not elapsed time. On the spring-forward day the local day is 23 hours (an hour is skipped); on the fall-back day it is 25 hours (an hour repeats). The actual elapsed time between two instants is unchanged.
When do the clocks change in the United States?
In the U.S., DST begins at 2:00 AM on the second Sunday of March (clocks jump to 3:00 AM) and ends at 2:00 AM on the first Sunday of November (clocks fall back to 1:00 AM). The European Union changes on the last Sundays of March and October, and many places do not observe DST at all.
What is a 23-hour day and a 25-hour day?
On the spring-forward date, clocks skip from 2:00 AM to 3:00 AM, so that calendar day has only 23 wall-clock hours. On the fall-back date, the hour from 1:00–2:00 AM repeats, so that day has 25 wall-clock hours.
How does DST affect calculating hours worked overnight?
For a shift crossing a DST change, count elapsed time, not clock labels. A shift over the spring-forward night is one hour shorter by the clock (23 real hours in the day), and over the fall-back night it is one hour longer. Payroll should pay for actual hours worked.