Other durations
About a half an hour alarm
A 30 minutes nap is on the short side of a power nap. You stay in light sleep, so you wake without grogginess and it still takes the edge off an afternoon. Lie down as soon as you press the button; the 30 minutes includes the time it takes to drift off.
30 minutes is 30 minutes, or 1,800 seconds. The alarm time updates live above, so the number you see is the actual clock time it will ring if you press the button now.
The alarm is created as a one-time alarm at the exact clock time, so you can see it in the list above, rename it, or switch the sound. Keep this tab open with the volume up; on desktop it rings from a background tab, on a phone keep the page in the foreground and the screen stays on automatically.
Prefer a countdown?
If you want to watch the time tick down rather than set a clock time, use the 30 minute timer instead. Both ring with the same sounds.