Kukuklok is amazingly easy to use; its home page (a very distinctive one) shows a digital watch face where you can set the time when you want the alarm to go off and a sound-selector to choose from: military trumpet, a cockerel, electronic alarm, classic alarm or the sound of an electric guitar.
Once you have set the time and sound, click Set Alarm, upload the chosen settings and don’t worry, it will go off!! Even if you lose your Internet connection!! So long as you don’t close your browser. Oh yes, and when the alarm sounds, you can either stop it or snooze for a further ten minutes.
If you need an online alarm clock, Kukuklok is the easiest one you’ll find.