Updated
The clocks changed overnight, and the next morning your alarm was an hour wrong. Either it went off an hour early and you lay there confused, or it stayed quiet an hour past when you needed it.
You did not set it wrong. The problem is that a clock change does not touch every alarm the same way, and the ones it misses look completely normal the night before.
Your phone changed the time, but not every assumption behind it
When daylight saving starts or ends, your phone updates its clock automatically, as long as the automatic date and time setting is on. That part usually works. What breaks is everything that was quietly counting on the old time.
A repeating alarm you set weeks ago was scheduled against the old offset. Most built in clock apps handle the shift correctly, but not all third party alarm apps do, especially ones that store a raw time and reconnect to a schedule in the background. The result is an alarm that still says 7:00, and still looks on, while firing at what used to be 7:00.
Check that automatic time is actually on
This is the first thing to confirm, because everything else depends on it.
On iPhone, open Settings, then General, then Date and Time, and make sure Set Automatically is on. On Android, open Settings, then System, then Date and time, and turn on Set time automatically and Set time zone automatically. If either of these is off, your phone did not move with the clock change at all, and every alarm is now shifted.
Watch the alarms you set by hand
Even with automatic time on, an alarm you created manually for a specific moment can behave differently from a repeating one. If you set a one off alarm for an early appointment the night before the change, open the clock app and read the time again after the clocks move. Do not trust that it looks the same. Delete it and set it fresh if you are not sure.
Repeating alarms are usually safer, but the morning right after a change is the exact morning you do not want to guess. It is worth opening the app once and confirming the next alarm is set for the time you actually mean.
The travel version of the same problem
The same thing happens when you cross a time zone. Your phone updates to local time, but an alarm set before you landed can carry old assumptions with it. If you are waking up in a new place, reset the alarm after your phone has settled on the local time, not before.
The part you cannot see
Like every alarm failure, none of this shows when you go to sleep. The alarm reads on. The time looks right. You only learn it is an hour off on the one morning the clocks moved and you had somewhere to be.
That gap is why we built Snora. Snora is an alarm you set like any other, and before you sleep it reads the settings that decide whether your alarm will actually go off, then shows one status: Safe, Caution, or Risk. It is a status, not a score to chase, and it is not a promise about your morning. The alarm and the check are free, and no account is needed.
Twice a year the clocks move and most alarms follow. For the ones that do not, the fix is the same. Confirm automatic time is on, and open the clock app once to read the next alarm before you close your eyes.
FAQ
Why is my alarm an hour off after daylight saving?
Your alarm is usually an hour off because it is still tied to the time it was set against before the clocks moved, and not every alarm app shifts it to match. Your phone updates its own clock when automatic time is on, but an alarm that stored a fixed moment can keep pointing at the old one while showing the same number. Built in clock apps tend to handle this correctly, and some third party apps do not.
Do phone alarms change automatically for daylight saving?
Most phone alarms change automatically as long as automatic date and time is turned on. On iPhone, confirm this under Settings, General, Date and Time, with Set Automatically switched on. On Android, it sits under Settings, System, Date and time, where both Set time automatically and Set time zone automatically should be on.
Why didn't my alarm spring forward with the time change?
An alarm can fail to spring forward if automatic time is switched off, or if the alarm app kept a raw time that never reconnected to the new schedule. Start by checking that automatic time is on, then open the app and confirm the alarm points to the time you actually mean. This blind spot is part of why we built Snora, which before you sleep checks whether the settings behind your alarm will let it ring, then shows a single status of Safe, Caution, or Risk.
Should I reset my alarm after the clocks change?
It is worth reopening and, if needed, resetting your alarm once after the clocks change, especially a one off alarm you set by hand for an early start. Open your clock app and read the next alarm time again instead of assuming it looks the same as before. If anything seems off, delete the alarm and set it again from scratch.