Other durations
About a 12 hours alarm
12 hours is about 8 sleep cycles of 90 minutes. If you want to wake at the end of a cycle, the alarms below at 12 hours and 13 hours 30 minutes are the closest cycle boundaries.
If you fall asleep in the usual 14 minutes, 12 hours gives about 11 hours 46 minutes of sleep, roughly 7.8 sleep cycles of 90 minutes.
12 hours is 720 minutes, or 43,200 seconds. In hours and minutes that is 12 h 00 min. 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 12 hour timer instead. Both ring with the same sounds.